Dungeons and Dragons uses an extremely abstract damage system. A hit for normal damage could mean:
- The target wasn't actually in danger, but is unnerved by the attack.
- The target dodged out of the way, and was missed entirely.
- The target parried or deflected the blow.
- The target was scratched by the attack.
- The target was injured by the attack, but not badly.
- The target was grievously injured by the attack, but manages to keep going anyway.
The only real observation the rules make on the issue is that (non-contact) poison triggers on successful damage, indicating that the skin has been broken in some way.
As nonlethal damage is damage for all intents and purposes, yes, nonlethal damage can cause bleeding.
Exactly what marks are left on the target when they are subdued is entirely up to the DM.
From my extensive experience watching TV, it seems like knocking someone out usually involves a fair amount of force (usually a swift blow to the head or upper body). Such a blow is going to leave bruising, although it might be somewhere out of sight (covered by hair, beneath clothes, etc.).
"Sleeper holds" and nerve-pinches are sometimes employed, which I would probably allow for grappling/martial-arts themed characters.
Characters explicitly trying to minimize visible bruising could do so with a circumstance penalty, which would increase the longer combat went on.