Here is the Oxford English Dictionary entry for blood knot, within "blood, n. (and int.)":
blood knot n. (originally) a multiple overhand knot of a kind formerly tied at the end of whips (or ropes used for whipping), to increase the pain of the blows inflicted; (now also) Fishing and Surgery a knot used to tie two ropes or lines together.
These knots are designed to draw blood, as an early example describes:
1863 London Rev. 24 Oct. 435/2 The novice proceeds by stages from tickling herself with a few strands of whipcord to goring her shoulders with hempen thongs full of blood knots.
The meaning switched over to a knot that joins two lines by the 20th century:
1935 E. Taverner & J. Moore Angler's Week-end Bk. 189 Under his critical eye, I joined the two lines with a blood knot. ‘I guessed you were a fisherman,’ he said.