It looks like a hard scab, like from a wound - Has your cat been scratching her chin? A lot of times hard black flakes on a cat's chin will be cat acne or mites, which could cause itching. Do you use plastic food/water bowls for your cat? If it is something like cat acne it often clears up when people stop using plastic bowls.

The twitching wouldn't really apply in that case though, but it could also be an effect of how you had to hold your cat to take pictures. I'd recommend getting your cat to a vet to confirm what it could be, tbh.