Is the fur worn off under the tail area or is this a breed that is clipped down?
How long has this been going on?
Is the skin on her abdomen darker too?
Is she spayed?
This may be due to a couple of problems.
One is her anal glands may be clogged resulting in the dragging you are seeing. You can read about this issue here
Dog skin does 'tan' dark gray or black in many dogs when exposed to the sun.
Dark skin on the abdomen can indicate a problem with a lot thyroid level which can trigger allergies and skin infections. Itchy skin folds around the vulva area can trigger scooting like you are seeing too. You can read about low thyroid in dogs here
Another possible hormonal issue that can cause a darkening of the abdomen and skin issues is called Cushings disease a problem with the adrenal gland. You can read about this here
For short term first aid you might want to see if some plain Benadryl helps with the itching. A common low dose is 1mg per pound of dog every 12 hours. Do check with your vet about using a med but this one or another may help her a lot. If you choose to use that please read here about cautions
You may also want to see if a bath (follow directions on the bottle) using a Chlorhexiderm shampoo from the pet supply store in case the problem is a bacterial skin infection will help. Malaseb is another good one to eliminate yeast from the skin.
But I think you need to have your dog checked out by your vet and perhaps some blood work done.
Hope this helps you!