Am I not allowed to have blood/gore in the game?

I think you can, the rule on the Jam Page says:

"Games with mature content are accepted when flagged. For disturbing or inappropriate content, we ask that jammers use common sense and flag this on the game page.

We reserve the right to remove any games that are not appropriate or flagged correctly, so please reach out if you have any doubts."

Either way, I'll definitely be using some blood.