I'm pretty sure your dorfs and other creatures are just tracking blood all over the place when it starts to rain and wash off whatever blood that was on them when they go outside.

That said, rain of blood does happen in evil biomes, but it shouldn't be happening in joyous wilds unless your embark straddles both types.

Ah, there is actually a weird bug with how terrain generates that can cause evil rain over a good biome! It's pretty rare though. Congrats? ^^'
https://dwarffortresswiki.org/index.php/DF2014:Weather
https://dwarffortressbugtracker.com/view.php?id=8781