Does this "dark" mean "thick in concentration" or "real black(dark) color"? I think it's the former, but the translation goes "black and red blood", so I doubt it.
ex)Doctors and nurses look at dark red blood during operations. If they wear white clothes, they will see afterimages of the red blood on the clothes....
The translation is wrong. The red is a dark red, compared to a light red.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.