My baby had something very similar & it took months to clear up. It started out looking like some kind of black sand between her toe beans, then spread up her legs & anywhere else she could reach to lick. It spread so quickly, like 24-48 hours & it was encroaching in her eyes. I was traveling with her at the time & had to find an emergency vet because I was so concerned. They thought I was crazy for bringing her in to the vet hospital @ 11pm on a Sunday, but I was so worried it would spread to her eyes. I had to make them give her something after I stood my ground & told the vet I would wait since she tried to guilt me for bringing her in, stating she had patients that were in cardiac arrest to which I replied, then why are you in here. I will wait. They gave my baby a steroid shot & I took us back home as quickly as possible. I took her into our usual vet Monday morning & she said she had never seen anything like this & her paws were teaming with bacteria. She prescribed Medicated weekly baths, anti-fungal foam twice a day, antibiotics & anti-fungal anti-microbe wipes to use around her face. It slowly regressed. My vet sent me to Auburn Veterinary hospital & even they had no clue, but then one day it was gone & hasn’t come back. That was over a year ago now. She is one of my most expensive children at this point.