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The black flakes sound like flea dirt. It may mean there are still fleas around. Try wetting a tissue or paper towel and rubbing it over your cat's fur so that the black flakes get on it. If there is a red tinge around black specks on the tissue, it is dried blood from the feces of the fleas and there are still fleas.
Weight loss in old cats (15 is old) can be caused by a lot of things.
The best way to get a diagnosis is to have your vet check out your cat and do some diagnostic blood work. A diagnosis will allow you to know what options for treatment are available and what is the best option for your friend.
Let me know if you have follow up questions.
I applied Frontline plus on my mail 10 year old cat