Here’a very quick mockup of an area.
Not particularly jarring, but it gets on many people’s eyes after a while (especially with areas like lab floors which would look like this:
This also demonstrates my other point rather well. How many people noticed the three blood spots to the left of the road? What about the two in the upper right? The two single spots in the bottom right? It’s very easy to lose data or not notice something that may be important when the field information is displayed with a colored background.
The alternative blood method (that of changing the background), is even worse, since floor, dirt, and grass will all look identical when covered in blood, easily leading to confusion.