My cat has started nipping at her hind quarters and seems to be itching. When I scratch her above her tail, I get these black specs under my nails. Any idea what this could be?

Fleas would be a good guess.

She could have fleas and the black specks are flea dirt (poop). Do you have a flea comb or brush of some kind that you can gently run through her fur to see what pops up?

It sounds like flea dirt. If you are getting that in your nails then she has a lot of fleas. I would take measures to get rid of the fleas.