Let me explain the board/general combinations with selected examples which are likely going to be the most active boards.

/b/a/: for artists who draw blood and hematomata
/b/c/: computer-generated images of blood
/b/i/: AIs generating blood
/b/f/ or /b/q/: requesting bloody files
/b/k/: for bloody oekaki
/b/o/: real life news or movies displaying blood
/b/r/: roleplays about blood
/b/u/: talking about blood and bloody things
/b/w/: for writers who want to write about blood

/g/a/: for community artists
/g/c/: CGI by and for the community
/g/i/: our future AI overlords
/g/f/ or /g/q/: community requests
/g/k/: for community oekaki
/g/o/: community news and blogposts
/g/r/: community roleplays
/g/u/: Gurochan discussions
/g/w/: for community writers

/m/a/: for artists who draw morbid and grotesque things
/m/c/: computer-generated images of things other than blood
/m/i/: AIs generating morbig and grotesque things
/m/f/ or /m/q/: requesting morbid and groteque files
/m/k/: for morbid and grotesque oekaki
/m/o/: everything you can find on 4chan's /gif/
/m/r/: roleplays about morbid and grotesque things
/m/u/: talking about things other than blood
/m/w/: for writers who write morbid and grotesque things

/d/, /x/, and /z/ are there for you to NEVER EVER see literal piss and shit, loli, or animals on the boards above.