That second character can also translate as Maelstrom, though. So, yeah, they could've called it the Black Maelstrom or Black Maelstrom Group, but I don't see the point. Especially since Maelstrom is probably the only translation of the second character that provides a distinctly nautical connotation.

Out of curiosity, are the other two groups called 不死団 and 双巳団?

ETA: Sheesh, I spend ten minutes looking up Hanja, only to have to change it to Kanji, and by the time I get done two other people have already made my point.