I assumed that the blue colour would be from internal bleeding (venous blood being apparent at the skin level), but:
> Diphenadione is a vitamin K antagonist that has anticoagulant effects and is used as a rodenticide against rats, mice, voles, ground squirrels and other rodents. The chemical compound is an anti-coagulant with active half-life longer than warfarin and other synthetic 1,3-indandione anticoagulants.[3][4] ... Rat poisons with diphacinone are often dyed bright blue to signal toxicity.[8]
> Diphenadione is a vitamin K antagonist that has anticoagulant effects and is used as a rodenticide against rats, mice, voles, ground squirrels and other rodents. The chemical compound is an anti-coagulant with active half-life longer than warfarin and other synthetic 1,3-indandione anticoagulants.[3][4] ... Rat poisons with diphacinone are often dyed bright blue to signal toxicity.[8]