With a fanbase as big as Harry Potter's is, there's never a complete consensus on anything, of course - but pretty much from the beginning "RAB = Regulus Black" was the frontrunner theory, with only small pockets of fans speculating anything else.
Take a look, for example, at this Mugglenet Article published only four days after the release of the HBP, which pretty prescient both in the general and specific details of RAB's identity:
Now that I have laid out the criteria I believe our mysterious R. A. B. has to follow, I will state who I believe the person is: Regulus Black, Sirius’ younger brother...
I think Regulus probably had more courage than Sirius gave him credit for. I have a feeling that Regulus found himself in a situation similar to what Draco Malfoy got himself into in HBP. Regulus probably did get scared, but unlike Draco, he refused to do what Voldemort was asking him to do and actually tried to help the future vanquisher destroy Voldemort. It takes a lot of guts to go against Voldemort, and I have a feeling Regulus knew he could never hide from Voldemort, and if he was going to die, he did not want to do it cowardly but was going to try to do something that would be devastating to Voldemort.
I think it is likely that Harry will come to figure out who the mysterious R. A. B. is through either Sirius’ house or Kreacher; both of which he has conveniently inherited. I think that Harry will again find help in a person that he was not expecting.
Or this 2006 Beyond Hogwarts article, which guesses not only at RAB's identity but almost every detail of how the locket plot would eventually go down:
So, where is the real locket now? Regulus' note says he intends to destroy it as soon as he can, but we don't know for sure that he did.
If it's anywhere, you'd suspect it's someplace in Grimauld Place, where we assume Regulus lived with his parents. In Harry Potter and the Order of the Phoenix, when they are cleaning out the parlor in Grimauld Place, a locket is mentioned, but only briefly... it's a good chance the locket Harry saw in the cabinet in Grimauld Place was the locket. But, where is it now?
Some fans believe Kreacher has it.
Even the people who argued against it did so mostly on the grounds that "it's too obvious" or "it'll be unsatisfying" rather than having alternate theories to propose.
Take, for example, this August 2005 article from a "RAB is not Regulus Black" supporter:
The idea that Regulus Black is RAB is by far the most popular conclusion and there seems to be lots of things that pop up confirming the theory.
Hands up, I don’t have ANY sort of argument against Regulus Black being RAB, apart from my gut feeling that it’s so obvious that it can’t be true.
Or this early 2006 forum post:
I never believed Regulus Black was RAB. That would be would be too much in your face approach, unRowlinglike. She puts something in your face at first but always debunks it afterwards
(And shoutout to this person, who while arguing against "RAB = Regulus Black" managed to randomly guess a pretty big plot point involving Kreacher):
No way is it Regalus. It just seems so... stupid and unlikley. Just because his initials fit... But i think KReacher was there and was forced to drink the potion, and that's why he is a little mad. It was too much for a House-elf to take.
A few places opened themselves up to fun alternate theories (I think, "RAB is Dumbledore, time traveling to the future" is my favorite) but none got much traction among the wider fanbase.
In the weeks and days counting down to DH's release, the theory went even more mainstream, showing up on multiple non-HP media sites. It's safe to say that by DH's release, the only people not expecting this twist were the ones who either didn't follow speculation at all or had a niche alternate theory.