Hi,
Is it a dark brownish/black sometimes waxy substance near the bottom of the hair shaft or on the skin? hard to remove, but if you wipe it enough with a warm wet cloth it starts coming off?
If so it could be yeast/fungal infection on the skin. my boys have this, one worse than the other. in their belly/underarm/thigh area. it doesn't come off with regular bathing. I use Malaseb shampoo and I have to use a brush in those areas. I use a softer bristle brush or toothbrush so as not to irritate the area more as it is tender there.
You can look up the Malaseb online, it also comes in wipes/spray/ and a rinse to use in between baths.
They have other shampoos/conditioners available for fungus too. Just google to find some. I bought the Malaseb in a package deal on Amazon.
Take care, teresa ;)
Welcome to the dog forum...
I personally do not have an answer, but check in from time to time, there are very informative people here w/great advice...
Most of luck to you, also here is "ask a vet" Dr. Cheng, you can also repost your concerns w/the vet here...