When my boyfriend scratches his flea bites and it bleeds the blood is black first then red and we want to know why?

This is normal. All blood behaves that way. Most blood from superficial cuts is venous, i.e. oxygen depleted -- and such blood is darker (although not truly black). As it is exposed to air, it absorbs oxygen and turns bright red. (If you ever seen blood in a plastic bag before being used for transfusion, it's very dark black-red, for the same reason -- low oxygen level.)