Technically, how does this function?Use at will. Your spell gains +4 DC, +4 CL (not subject to DC increase), and you take 3+ (D8 DMG per spell level).
I loaded a build up in the latest JEGs with the Feat, put the Feat on the Hotbar, selected it, got the Log response of "Warning: Blood Magic is outlawed and detested in most civilized areas," then cast a spell = no HP was deducted.
I used many different spells, both attack and defense, with no HP deduction, nor any perceivable difference in DCs with Blood Magic active or not.
Is JEGs broken, or is there some special way to apply Blood Magic to spellcasting?
All spells I think.