Hi there delish. Here's a thread
http://www.bdsmlibrary.com/forums/sh...ad.php?t=14037 that talks a little about marking...
In general, bruising occurs when blood capillaries under the surface of the skin are broken. This can happen with the force of the blow, or the pinpoint of, say, a needle, because the blood rushes into the area to "plug the wound." Bruising isn't necessarily dangerous, but it is something to think about before doing. Bruising is your body's visible way of telling you it was "injured"--bruising requires various wound healing mechanisms of the body & thus takes resources away from other areas. If you are diabetic, have a heart condition, etc, bruising intentionally can be pretty dangerous, so definitely take extra care.
A lot of people say "Oh, I absolutely love the bruising" but repeated & prolonged bruising over one area of your body can have a marked effect on your body's attempt to wound heal, and eventually, can lead to more severe bruising and issues with your health.
Advice? Play hard, but play safe. If you are intentionally bruising, be VERY VERY VERY careful--what it takes for 1 person to bruise isn't what it takes for another. Some people would require an unsafe amount of hitting to get the "desired result". Also, some people don't bruise for 24-48 hr after play.
If it hurts, stop. You've probably done enough.
Good luck, play safe!
delia