Blood Party is hipster slang for a satanic ritual sacrifice. All the cool kids are doing it.
Blood Party is a rave where everyone shows up painted red, making it look like they're covered in blood.
Blood Party is that few minutes after you cut yourself badly, and before the ambulance arrives, where you're getting increasingly light-headed and start playing with the blood instead of holding the compress on the wound. Bonus points for writing on the walls in blood.
Blood Party is a new third party in national politics, who are campaigning on a platform of either increasing blood donations or enforcing some kind of white supremacist "racial blood purity"; the name was a cause of some confusion and drew both groups. Also, "blood party" is descriptive of their first campaign rally where these differences were discovered.