Blood... happy thoughts... happy thoughts... bella... bella... phew.
It depends on what kind of animal you're talking about.
Vertebrates have hemoglobin in their blood which makes it red. Depending on how much oxygen is in the blood, it can range from bright red to dark red. That's why when you cut your finger, its always bright red because, bam, oxygen. But hemoglobin blood never turns blue. That is just a lie. Some of your veins look blue but that is for different reasons.
A lot of molluscs, on the other hand, have a greyish-whitish-yellowish blood that turns blue when it is exposed to oxygen. Since they usually live in cold, low-oxygen places, their blood isn't usually blue until it exits their body, like in the event that you cut one of them in half with a butcher knife.
On Star Trek, Spock has green blood. That's weird but maybe its possible. It looks like lime snow cone flavoring though so sometimes I doubt its real, but its hard to tell on TV because you can't really taste it or anything.
