The chytrid will often have them doing multiple sheds....the theory is that they are trying to rid themselves of the infection by shedding it off.
If the shed was dark in color, it may be that the skinn held some dark melanin and it is common. When snakes shed, you can sometimes see the colors & patterns on the shed skin.

It would do no harm if you treat the frog for it just to be safe. But if you are seriously concerned, then a visit to a qualified vet is in order.