and the red hp bar is the current damage delt in the "combo"
once you are able to successfully use moves again the red damage bar will be dropped, its just a visual to show the current damage delt to a player in quick succession
the black hp bar isnt black, just transparent, black and red both are lost damage, while red is ^^^ black is just total damage (red goes over black damage until red is dropped, then the hp bar shrinks down showing more of the background)