beacuse Ive heard of something called 'blood magic'

Their spells are called night powers and blood magic, but you're still using magicka. I think they call it that to emphasize that the spells are not linked to a school.

its confirmed that they do, if you pay attention to the gameplay from E3 you can see him using mana when casting

They get separate magicka, stamina, and health reserves though. So if your character has never put a single perk or level into magic they can still be epic vampiric mages.

they dont get seperate , there is one perk that adds 50 to each while in that form

I think the "Blood Magic" actually uses your health to cast. The E3 videos showed the Vampire Lord's health decreasing as he cast certain spells like Vampiric Grip.