I have a boxer mix she is a wonderful dog. I am having so many problems with her skin. She sheds so bad and when I sweep the floor where she had been it looks like hair and sand it very bad. Her underbody had turned black and she has big warts all over her. I was told by a vet to give her fish oil this hasn't helped at all , we bather her and the smell and all comes right back. What can this be.

I will not diagnosis your dog but will say it almost sounds like what my dog has. Allergies. I would go back to your vet and explain you want to find out what is up. They may change your dogs food to see if that helps, give some fatty acid supplements, or medications.
It also could be something totally different. We can not see your dog and can not diagnosis over the internet. That is not fair to you or to your dog as we would be guessing.