Part of the problem in this matter is that gladiators were different in status and purpose from each other. There's a whole world of difference from a doomed prisoner-of-war pushed into the arena with soiled underpants as opposed to an experienced professional contract fighter. Yes, blood was involved. We all know that and that was part of the religious side of things. Romans wanted a good fight. They wanted to see skill & courage. But they usually dictated whether these men could live or die if defeated. It was a catharsis for the common people who were otherwise powerless; now they could condemn a man to death for cowardice, clumsiness, or simply because he had cost them too many denarii in bets. However, as someone said earlier, perhaps as little as 10% of one-on-one contestants died. Therefore blood was less important than enjoying a good fight (not to mention the cost of replacing a dead gladiator)
I think blood became more frequent as time wore on. As fights became mundane, something had to raise the excitement level. I seem to remember that Augustus banned fights without mercy, but didn't these come back later on?