Welcome to the forum.
You shouldn't be at all worried about STDs from this event. First, fingering never transmits any infection of significance, as far as we know. Second, visible dirt does not imply increased infection risk. Visbibly very clean hands have just as many germs, and the same types of germs, as dirty looking ones.
So this doesn't suggest high risk for any infection. My advice is to put it out of your mind and not worry about it.
Best wishes-- HHH, MD