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As with any change, good or bad, it may take him some time to get used to it. Though some never do.
If after a week he's still black bearding it, I'd say he doesn't like it in there.
I've also noticed that he's been head-bobbing and black bearding a lot when it wasn't even in there, he puffs his beard out, and when I put my hand in and hold him he goes so calm again, does he want to be held or can he see himself?
That could be his reflection. i recommend getting a background from the pet store. i got a canyon and it helps. my beardie chases his shadow on the wall when i let him out.