Hi Rimiu
IMO Bloodmage is a good pick. He looses out on Empower, which can be very potent, game changing even - but once per fight and several times per rest only. Granted he also looses access to the ability to spend this Empower to renew half of Assassin Guile resources - therefore more stealthiness. However a Bloodmage has access, in practice unlimited, to best spells to deal with each situation and that is invaluable. Also at low spell levels can stack all meaningful buffs AND STILL be able to use other spells from these levels - repeatedly so -> particularly in early game (which is the most difficult part), this is game-changing.
And Corrosive Siphon is an awesome low level health drain, so healing really isn't a huge concern for a Blood Mage when there are several enemies to affect them with.
Good synergies... well, first of all, Arkemyr's Dazzling Lights spell from level 1 is very nice for such a character. Cast it from stealth near the enemies to attract their attention and cluster together in a tight group waiting for aoe bombardment - but watch out not to hit any enemies with the Lights, or combat will start and you will be revealed!
Then certainly try to get Ninagauth's Teachings grimoire - its unique level 4 spell, Ninagauth's Shadowflame, is perfect as an opener after Dazzling Lights. Not only does it similar damage to a fireball (actually a bit higher) but cold-based, BUT ALSO Paralyzes enemies on hit. With the Assassinate +25 Accuracy bonus, this usually hits! So now you start combat with all/most enemies in a tight cluster and unable to move or act... AND subject to an additional 25% Hit-to-Crit conversion.
Also in Turn Based mode Ninagauth's Death Ray, another exclusive from this grimoire is very good for such a build, as it ticks 3 times, deals high unresistable damage in a line aoe, if enemies don't have you in their sight cones, you actually remain stealthed after the first hit for a second Assassinate tick! Nice to have someone with Morningstar modal to lower enemy Fortitude, though. Significantly worse in real time, as it has 11 far weaker ticks (1 per second), most of which wouldn't benefit from Assassinate. Still might be useful to crack enemies with extreme Armor Rating due to unresistable Raw damage type.
Next Deltro's Cage helm. You need to either steal it or kill an influential figure for it. It's a MAJOR force multiplier for such a build. When hit with lightning, it stores its energy and for a short time provides a % damage lash equal to the number of hit points you lost when lightning struck you. Meaning if you target your stealthy Assassinate Chain Lightning on YOURSELF and manage a 150 damage critical (doable... usually when you're low on HP... or when doublecast from Vithrack Slippers triggers :P), your attack, including spells, will now do 250% of their original damage! Sometimes its only 30-40 damage / % lash (on Graze), often ~60-80 / % lash (on Hit). But if you stack Critical chance (and avoid raising Reflex defenses), 110-140 Assassinate crits / % lash are also fairly common. What's interesting that the follow-up bounces of this initial Chain Lightning (which strike the enemy now), will already also carry this % lash....
Before Chain Lightning you can use the early Shocking Grasp for a much weaker, but still quite noticeable lash.
For longer fights you'll be able to extend this (and other helpful) effects indefinitely with Wall of Draining.
Would be nice to use Devil of Caroc's Breastplate, as for a large part of the game it will allow +1 use of stealth (trough Shadowing Beyond) per encounter. And that's valuable. So make sure you import/configure a background history that accommodates its availability. There are other ways to become invisible (including a unique spell from some exclusive grimoires), but they usually involve a Recovery period, so are not as nice to act fast and furious. Could be great for solo-play, though. Assassin Slippers can be nice 1/combat, but in Turn Based mode at least, their effect often went to waste.
Be a Human with Fighting Spirit and maxed Con. As a Bloodmage you can self-sacrifice to ensure you're always under 50% hitpoints and Fighting Spirit is permanently active (and the bonus is real good: +7 Accuracy, +15% damage). I also like high Per for Accuracy and Int for aoe sizes and effect durations. However in real time Dex shouldn't be neglected either. Might can be left around base (it also increases the damage you self-inflict), while Res dumped.
Other great spells include Ninagauth's Freezing Pillar (almost impossible to use while remaining stealthy due to loudness, but still offers very high damage over time) and Minoletta's Piercing Burst - I was using this 80% of the time after I got it (15% probably went to Death Ray for unresistable damage and 3 ticks) - high damage, party friendly aoe, high Penetration.
Before that the Blightheart arquebus is nice for its 10% corrosive lash that applies to spells.
Of course, stack Critical damage and chance as much as possible: Dirty Fighting 10%, Ring of Prosperity's Fortune up to 15%, Merciless Gaze 15% Uncanny Luck 5% (or 10% with Community Patch - its maybe not worth it without this mod), The Third Eye 10%.
After early game, try to always use Crusted Swordfish food when resting, it will help a lot. It can be crafted or bought in Periki's Overlook in Neketaka.
Crowd Control spells can also work very well with the Assassinate bonus (+25 Accuracy!). But since it's such a limited commodity, I used homicide as the ultimate form of CC.
Oh and don't worry about seemingly short duration on some spells, (particularly buffs like Merciless Gaze). With Power Level bonuses and Int stacking they will have a very respectable duration before long. Note that Ray spells and Walls have a fixed duration, though.
Kinda late game, but I enjoyed using Engoliero de Espirs estoc with this build. It triggers its Ghost Blades cone aoe attack on kill - regardless of kill method. These get boosted by the various Assassin bonuses and ranged bonuses (sneak attacks, Deathblows, pet with +10% damage to ranged attacks and other) and can be enchanted to return as much health as they deal damage. This can lead to nasty kill cascades on damaged afflicted enemies and a lot of healing for you.
Something like this can happen: