The blood of flies, like most insects, is clear or a pale yellow or green. This is because insects have open circulatory systems, where their blood, called hemolymph, mixes with the fluid surrounding their tissues.
The red stain you might see when you swat a fly isn't actually blood, but rather pigment from the fly's eyes. Flies have a compound eye that contains various pigments, which can be released when the fly is squashed.
