Others have given the rap beef context, but nobody has mentioned why a blood would have been on stage or why that was significant. At the end of the concert, Kendrick got a bunch of people from a bunch of LA gangs on stage and made a big show of unity.
TMZ article - mainly as a source, the article itself isn’t fantastic

Disagree. I’m not from the US, so maybe guns being seen as fun is more of a cultural issue in not aware of, but plenty of weapons are seen as fun here in the UK. And while we do have gun crime, I think firearms are seen differently here vs the US.
People will go axe throwing for fun. With Scouts, we shot air rifles on camp a number of times (supervised by people who knew what they were doing). Knife throwing is also a thing. Clay Pigeon shooting is also not uncommon here.
I genuinely believe using a weapon on a (non-living) target is fun for a lot of normal, well adjusted people.