Was the blue fire from Crimes of Grindlewald Fiendfyre?

Question

In Crimes of Grindlewald, Grindelwald uses a powerful blue flame that takes on the shape of monsters. Was this Fiendfyre?

Answer 1

Grindelwald’s spell is different than Fiendfyre.

The spell Grindelwald used to create his circle of protective fire doesn’t seem to be Fiendfyre. When Fiendfyre burns, it seems to burn indiscriminately - Crabbe casts it in the Room of Requirement but it shows no loyalty to him. It tries to burn Crabbe, Goyle and Draco just as much as it tries to burn Harry, Ron, and Hermione.

“Like it hot, scum?’ roared Crabbe as he ran.

But he seemed to have no control over what he had done. Flames of abnormal size were pursuing them, licking up the sides of the junk bulwarks, which were crumbling to soot at their touch.”
_{^{- Harry Potter and the Deathly Hallows, Chapter 31 (The Battle of Hogwarts)}}

Crabbe’s Fiendfyre ends up killing him.

“Fiendfyre – cursed fire – it’s one of the substances that destroy Horcruxes, but I would never, ever have dared use it, it’s so dangerous. How did Crabbe know how to –?’

‘Must’ve learned from the Carrows,’ said Harry grimly.

‘Shame he wasn’t concentrating when they mentioned how to stop it, really,’ said Ron, whose hair like Hermione’s was singed, and whose face was blackened. ‘If he hadn’t tried to kill us all, I’d be quite sorry he was dead.”
_{^{- Harry Potter and the Deathly Hallows, Chapter 31 (The Battle of Hogwarts)}}

Conversely, Grindelwald’s spell lets people who are loyal to him past the fire unharmed.

GRINDELWALD
Protego diabolica.

He spins and draws a protective circle of black fire around himself. The exits close.

ABERNATHY, CARROW, KRAFFT, MACDUFF, NAGEL, and ROSIER walk through the flames into the circle.
_{^{- Fantastic Beasts: The Crimes of Grindelwald (The Original Screenplay)}}

Grindelwald’s circle of protective fire only burns those who aren’t truly loyal to him.

ANGLE ON KRALL, hesitating.

Then he decides the circle is the better option, braces himself, runs into the fire—and is consumed.
_{^{- Fantastic Beasts: The Crimes of Grindelwald (The Original Screenplay)}}

Therefore, it’s not the same thing as Fiendfyre, since it doesn’t have the same effect once cast. It may be a similar spell, but because of the different effects, it’s certainly not exactly the same as Fiendfyre. It’s possible they’re separate types of one sort of spell, but Grindelwald’s spell is distinct from Fiendfyre.