It's a reference to the first of the ten plagues, where God turns the water of the Nile to blood. Similarly, this puzzle turns instances of WATER into the blood groups (A, B, O, and AB).
One of the plagues of Egypt was turning water to blood, and each theme clue leads to a phrase with WATER, but its entry replaces "WATER" with a blood group (A, B, O, AB): [WATER]-BASED -> [A]BASED; [WATER]LOGGED -> [B]LOGGED; STANDING [WATER] -> STANDING [O]; [WATER] STAIN -> [AB]STAIN.