Bloodmagic doesn't work with MP Absorb. It once did but it was long nerfed into oblivion.
MP in bloodmagic will be added like HP such as 7500 MP and the MP on hit acts like HP on hit. Spells cost double as you knew and at 90% HP missing , you can deal 180% more DMG. Also MP% acts like HP%.
That means high MP% increases the bloodmagic alot and combined with HP% together. Balance legend affix is great for this too.
Can be used for PvP . If bloodmagic still worked with MP Absorb, it would be OP .