I think you should just stop using the device. The skin changes you describe are probably the result of minor trauma to the skin. This can't make your poenis any bigger, but it also can't cause permanent damage. The dark discoloration may take several weeks to fade, but if you don't rub or traumatize it further, your penis should return entirely to normal.
Penises are supposed to feel tight when you have an erection . . .
Best.
Dr. Rockoff