I know you can't easily do it the other way but is it ok to go over masonry paint with bitumin paint?
I want to paint the bottom of the wall black and protect it because water sits against it after heavy rain
Bitumen will age and dull very quickly in an exposed state, would be inclined to use a quality black gloss paint.
If the emulsion is sound, can't see a problem. Use Zinsser Bullseye 123 as a adhesion primer over the masonry paint and then Dulux Weathershield undercoat and High Gloss.
You are much better fixing the reason why water is sitting at the bottom. You are potentially going to cause more problems by not letting moisture soak out through the wall once you have painted bitumen on it. As a minimum you can expect damp and you may find that the face of the bricks "blows" when it freezes