This has been a hot debated topic for a long time and Flamboyant-era "all magic is grey" ruling has done irreparable damage to many things to this day..
Blood magic: Uses your blood and the most effective method is by forcibly tearing blood out of other people. And then has rituals that have an in-built always failure chance that could tear apart areas of the map or release untold horrors onto the mortal plane. Additionally, even if it was 100% safe in terms of risk to the caster, it has massive potential to be used for evil intentionally too, more than other magics. The cherry on top is that its a magic that can forcibly tear a hole into a god's private personal vacation home plane, which means most gods by default would probably back up calling blood magic a dark magic.
Basically, blood magic is a dark magic. Some deities don't care as much about it because there isn't as much forced moral implication to its use, but some still exists and that which does is definitely leaning to dark!