In the film, Fantastic Beasts and where to Find Them, Credence, the Obscurial, appears in the first shot as an invisible force causing destruction throughout the city. Then, later in the film, we see him attacking Mr. Shaw's son, as an invisible force, again. Then in the later shots, he is a great black mob, or force. Why does Credence not always appear as a visible Obscurial, and why does he turn invisible, and how?
The Obscurus only seems to be visible after it’s let to ‘take over’.
The Obscurus only seems to become a visible black mass rather than an invisible force after Credence decides to stop trying to control it and it ‘takes over’ him.
(whispers, making eye contact finally)
But I don’t think I want to, Mr Graves.
The Obscurus moves horribly beneath CREDENCE’S skin. An awful inhuman growl comes out of his mouth, from which something dark begins to bloom. This force finally takes over CREDENCE, his whole body exploding into a dark mass which hurtles forwards out of the window, narrowly missing GRAVES.
GRAVES stands, watching as the Obscurus zooms out and over the city.”
- Fantastic Beasts and Where to Find Them: The Original Screenplay
It’s possible that his choice to ‘let it take over’ also let it become visible, because it was in control and given free rein. In addition, it may have become more visible the more powerful it got.
It’s also possible that it’s defined by the debris it creates.
Dan Baker mentioned an idea for showing the Obscurus, though it’s unclear if it was the final idea that was actually used in the movie or just a concept thought of but not used, was to have the Obscurus invisible at first, and then become visible because of the debris it creates.
“ANOTHER STUDY OF THE OBSCURUS
At first it is invisible but it is eventually defined by the debris from the mayhem it has created on New York’s streets. DB”
- Art of the Film
If this was indeed the final concept used, then that would be why the Obscurus becomes visible.