The story
From blood + bath, the latter used referring to a metaphorical deluge. Compare West Frisian bloedbad (“bloodbath”), Dutch bloedbad (“bloodbath”), German Blutbad (“bloodbath”), Danish blodbad (“bloodbath”), Norwegian Bokmål blodbad (“bloodbath”), Swedish blodbad (“bloodbath”), Icelandic blóðbad (“bloodbath”).
Etymology from Wiktionary, CC BY-SA 4.0.