Turtle's shells are made of a hard material, like your fingernails. It would not bleed, just his fleshy parts can bleed.

He's a red-eared slider, right? How old? Could it be just a coloration change? I know when our turtle bumped her shell very hard, it just left a white mark, not red. She was a painted turtle, and had a very dark green shell.

Does your turtle have a place to climb up and sun himself and dry out when he wants to? This is important for shell-health, from what I remember ...