Our kitty has ear problem

Hello.

Our new kitten has always scratched behind her ears, ever since we got her. Lately, she's been scratching a bit more. She's been to the vet and everything, and on her very first vet visit, she had clean ears.

I recently was petting her and found something rough behind her ears. I pulled out some loose hair, and with it came tiny black crumbs.

I'm not sure what it is, if it's the reason she's scratching, but has anyone had this problem with their cats? What can I do?
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