Is the title "Dark Lord of the Sith" reserved for the Sith Master?

Question

For the Sith Order under the Rule of Two (i.e. from Darth Bane to Palpatine), does the Sith Apprentice hold the title "Dark Lord of the Sith"? Or is the title reserved for only the Sith Master?

In the (Legends) Bane trilogy, it appears to be reserved for the Sith Master. For example, from Dynasty of Evil:

That was why Bane had chosen Zannah as his apprentice: she had the potential to one day surpass even his own abilities. On that day she would usurp him as the Dark Lord of the Sith and choose an apprentice of her own.

Later, Darth Zannah affirms that the Apprentice becomes the Dark Lord upon killing the Master:

The Master will train his apprentice in the ways of the Sith, until one day she must challenge him. If she proves unworthy, the Master will destroy her and choose a new apprentice. If she proves the stronger, the Master will fall and she will become the new Dark Lord of the Sith, and choose an apprentice of her own.

However, Darth Vader -- who never became the Sith Master -- appears to hold the title. He is referred to as the "Dark Lord" in the movie scripts, as well as in the canon novel Tarkin (albeit always from the perspective of Moff Tarkin, who is not a Sith). Probably on this basis, Wookieepedia's canon article on the title says that the title was "shared" by the Sith Lords under the Rule of Two.

There appears to be a contradiction, in that the Bane trilogy reserves the title for the Master but the canon sources do not. How is this explained? Did the Sith Order start giving the title to the Apprentice sometime after Darth Bane and, if so, when? Is the Bane trilogy simply wrong to reserve the title for the Master (it's Legends, so canon supersedes it)? Or did Darth Vader not really hold the title?

The last option (that Vader didn't really hold the title, but was publicly assumed to hold it) seems possible for several reasons:

1. Vader is never called the Dark Lord by Palpatine, and never calls himself the Dark Lord. He is only referred to as the Dark Lord from an out-of-universe perspective (e.g. in the scripts) or by non-Sith characters (e.g. Tarkin) who wouldn't know his true title(s). Within the Sith Order, it's possible Vader doesn't really hold the title.
2. Palpatine was not known to be a Sith by the general public, so from the public point of view Vader was the only Sith and therefore the Dark Lord.
3. Out-of-universe, there may be a bit of a retcon here: Lucas didn't initially intend Palpatine to be a Sith and started calling Vader the Dark Lord in the Episode V script; when the Emperor's character was changed to be a Sith, he was stuck with Vader's title.

Answer 1

TL;DR: No.

Canon:

In Disney canon, Vader is referred to as "Dark Lord" many times, but usually without "of the Sith" added.

In the four canon novels mentioned below (Tarkin, A New Hope, Empire Strikes Back, and Return of the Jedi), "Dark Lord of the Sith" is used six times, "Dark Lord" is used 98 times. All 104 of these instances refer to Vader, not Palpatine.

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Tarkin:

Tarkin didn’t always agree with Vader’s methods for dealing with those who opposed the Empire, but he held the Dark Lord in high esteem, and he hoped Vader felt the same toward him.
- Star Wars: Tarkin

...

Later, observing the rapport the Dark Lord shared with the stormtroopers who supported him, and the technique he displayed in wielding his crimson lightsaber, Tarkin grew more and more convinced that his suspicions were right.

Vader might very well be Jedi Knight Anakin Skywalker, whom Tarkin had fought beside during the Clone Wars, and for whom he had developed a grudging appreciation.
- ibid

There are 14 other uses of the phrase "Dark Lord" in this book, none of them including the words "of the Sith", and all of them referring to Vader. This is relevant because Tarkin basically knows that Palpatine is also a Sith:

Tarkin had his private thoughts about the Emperor, as well. That he and Vader were kindred spirits suggested that both of them might be Sith. Tarkin often wondered if that wasn’t the actual reason Palpatine had been targeted for arrest or assassination by the Jedi.
- ibid

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A New Hope:

However, the full phrase "Dark Lord of the Sith" is also applied to Vader in canon - in the novelization of Star Wars: Episode IV: A New Hope:

Flowing black robes trailing from the figure and a face forever masked by a functional if bizarre black metal breath screen — a Dark Lord of the Sith was an awesome, threatening shape as it strode through the corridors of the rebel ship.

Fear followed the footsteps of all the Dark Lords.
- Star Wars: Episode IV: A New Hope

This is the only use of the full title "Dark Lord of the Sith", but there are another 18 uses of the phrase "Dark Lord" in the book, all of them referring to Vader.

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Empire Strikes Back:

And the novelization of Star Wars: Episode V: The Empire Strikes Back follows suit:

Towering above them in his black cloak and concealing black headgear, Darth Vader, Dark Lord of the Sith, entered the main control deck, and the men around him fell silent. In what seemed to be an endless moment, no sounds except those from the ship’s control boards and the loud wheezes coming from the ebony figure’s metal breath screen were to be heard.
- Star Wars: Episode V: The Empire Strikes Back

...

Admiral Piett felt great awe as he stepped into the dimly lit room and approached the Dark Lord of the Sith. His master stood silhouetted so that Piett could just barely make out the lines of a set of mechanical appendages as they retracted a respirator tube from Vader’s head. He shuddered when he realized that he might be the first ever to have seen his master unmasked.
- ibid

...

ONLY one being in the entire universe could instill fear in the dark spirit of Darth Vader. As he stood, silent and alone in his dim chamber, the Dark Lord of the Sith waited for a visit from his own dreaded master.
- ibid

...

“You are beaten,” the Dark Lord of the Sith gloated. “It is useless to resist. You will join me or you will join Obi-Wan in death!”
- ibid

...

“There is no escape,” the Dark Lord of the Sith warned, looming over Luke like a black angel of death. “Don’t make me destroy you. You are strong with the Force. Now you must learn to use the dark side. Join me and together we will be more powerful than the Emperor. Come, I will complete your training and we will rule the galaxy together.”
- ibid

These are the only five uses of the full phrase "Dark Lord of the Sith" in this book; there are 36 uses of "Dark Lord". All 41 of these instances refer to Vader.

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Return of the Jedi:

The novelization of Star Wars: Episode VI: Return of the Jedi doesn't use the full phrase "Dark Lord of the Sith", but all 25 uses of the phrase "Dark Lord" refer to Vader.

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Legends:

Limiting ourselves to one Legends novel - Star Wars: Shadows of the Empire, we find 14 uses of "Dark Lord of the Sith", and two uses of "Dark Lord". All 16 refer to Vader.

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Conclusion:

We don't know what previous Sith would have thought of it, but we can clearly say this much: At least during the Imperial period, the title "Dark Lord of the Sith" was exclusively applied to the apprentice (Vader) rather than the master (Palpatine). This might mean Vader was actually entitled to the honorific "Dark Lord of the Sith", or it might mean people thought he was the Dark Lord of the Sith because they didn't know that Palpatine was also a Sith.

Answer 2

The title "Dark Lord" is only reserved for the master. Or the head of the order like back in the days of Revan. In Vader's case he was addressed that due to ignorance. As you would remember people of the Empire downplayed the Force as if it didn't exist. Even when faced with undeniable proof.

Edit:

"The Sith must be ruled by a single leader, the very embodiment of the strength and power of the dark side. If the leader grows weak, another must rise to seize the mantle." Darth Revan via Sith Holocron

The title Dark Lord of the Sith or Jen'ari in Sithese, originally referred to the ruler of the Sith Empire and later used by the Order of the Sith Lords.