for a blood mage i personally find constitution to be more important than virtually any other builds since health becomes a way to regenerate spells, so the extra health cushion can be useful. (But i am playing on potd + skaen/ondra/wael/berath challenge, which means i have no information on my specific health (wael) and also knockouts become lethal after 10 seconds (berath), so the convenience of the extra health cushion for me might be specific to that set of challenges.)
might is particularly bad for blood mage since IIRC, might will increase the raw self-damage. as a part rogue you will already have lots of damage boosts for martial sneak attacks (and eventually deathblows) anyway, so balancing that out with perception is more important than having a wee bit more might.
move might points elsewhere (but don't go below 10). again keeping in mind that my recommendation for con might be influenced by my challenges, i would personally recommend moving might and possibly even some perception into con (a blood mage can spam eldritch aim as needed if accuracy is a problem).
for a simple intro to how a blood mage can play, i would pick up chill fog, eldritch aim, and then at AL2 concelhaut's siphon. overlapping chill fogs to ensure constant blinds (for sneak attack and general debuffing utility), eldritch aim to max accuracy for both sneak attacks and chill fogs, and then concelhaut's siphon spamming to keep your health up should give you a nice preview of how powerful a blood mage can be. If you want an easy time for early potd fights, get slicken instead of chill fog (and just use chill fog when you get a grimoire) and just repeat spam slicken (plus optionally eldritch aim to help land) until everyone who's not a floating creature is constantly tripping and you can just pick them off at your leisure.
Edited by thelee, 07 March 2019 - 12:38 PM.