I wonder though if the inconsistencies of blood color in a scene are more about cameras and monitors involved than the actual color of the prop?
Let's just say you can change the impression a lot in post-production, so it's probably a combination of both - much of what you see in digital-age movies has been color-filtered for mood (sometimes that's depressingly obvious - army faces superior foe - heavily de-saturated colors, almost greyscale, the remaining color palette is blue-shifted to make the scene cold - leader gives rousing speech - colors return and are shifted towards red to give the scene a warm glow).
So if there's blood visible, that'd affect it a lot.