Some black rats don't turn all black...they sometimes have some hinting of their youthful color pattern (white or greyish). I caught a neat adult male that had red residual patterns on his back. He was really neat lookin'. I couldn't even venture a guess as to how many black rats I caught in the same area (lots) and he was the only one with the red in the pattern. I never found a
corn snake ever... The red was
corn snake like red.
On a side note: Of all the black rats I caught, I can only remember two or three that showed defensive behavior when handled nicely.....the red one was one of them. After he was picked up, he would calm down. When I put him on the ground and tried to pick him up again, he would instantly become defensive.
I don't know for sure what he looked like as a young one, but I would guess it is as you are describing the one you found.
Show us some pictures. I would love to see it.