bloodmage + arcane warrior is probably one of the best things you can do

you don't lose health (being an arcane warrior makes you nearly invincible at the end of the game ^^)
and bloodmagic will allow you to cast spells, because as an arcane warrior 80~90% of your Mana-ressources will be used by passiv abilities, hence you have a really weak mana-pool.. the bloodmagic tree will help you to leech lifeforce instead of Mana when using spells

be advised if you use a sword, your character will sheath the weapon when it tries to cast a spell

there are only a few exceptions,
like
arcane bolt.
the whole spirit-spells tree
death-cloud + curse-tree
chain-lightning + inferno.
I believe the glyphes will work too without the weapon to be sheathed..

So my suggestion is to spend ~80% or even more of your points in "Magic" and
go for "Flaming Weapons" and "inferno" in the fire tree (flaming weapons will add a helluva lot fire dmg if you spend enough points in the Magic attribute... trust me
you'll probably recieve a +30fire dmg bonus or even more very early which will allow you to kill normal enemys *very* quickly.

together with Rock-Armor and Arcane Shield which will provide a great deal of defense and armor (2 different things) and the Arcane-Warriors "Fade Shroud " skill u'll barely get hit

the bad thing however is -> you might have problems getting the attention of multiple enemies at the same times so you will either need a *real* tank OR a mage with a lot of crowd-controll abilities.

btw there is a nice mod
http://www.dragonagenexus.com/downloads/file.php?id=14

feel free to try out speccs or spells or (the reason why I think this Mod is really helpfull)
to respecc your party...
It's really annoying if you play a tank yourself and want to play with Alistair in your party.