Your level of defense has been discussed fairly well already. If you haven't done any normal instances, I'd work your way into those first. Once you've done a few and are comfortable, you can walk into some of the easier heroics - even if you aren't at the "cap".
If you have some time to quest, don't forget about the Axe of Bloodstained Ice - Item - World of Warcraft. You'll take a bit of a DPS hit, but that should put you high enough to be uncrittable in heriocs.
It's part of a long chain, but on horde side I know it starts at Koltira Deathweaver (so Alliance side it's probably Thassarian, or whatever his name is).
As far as specs... you can tank OK in heroics as blood. Your talent selection looks pretty good, though I'd try to find some way to get Death Rune Mastery somehwere in there for more flexibility (i.e. "oh, they pulled an extra pack, let me drop a DnD or a pestilence to pick them up"). I'd probably drop Bloodworms to do it, but that's more personal preference than anything.
Just be aware that until 3.0.8, your AoE will be limited (DnD, then IT/PS/Pest, obviously), and not as high as that of a Frost or Unholy build. When dealing with "AoE first, blame the tank later" puggers, it'll probably be a bit of a headache - and they'll probably be a little upset if you keep asking them to CC.
As a blood spec, you may be perceived as more "squishy" than a Frost/Unholy tank. Due to the reactionary nature of Blood (i.e., it deals with returning health to you, which has to occur AFTER you get hit), you'll occasionally spike and make the healers freak out. When you're missing frost's extra avoidance, and Unholy's extra Damage reduction, healers may perceive you as "too squishy". I would suggest starting off with a healer you know and trust, and get feedback.
This will likely change with the patch, but I'm just letting you know of this so you can be aware.
Luck is what happens when preparation meets opportunity. -Seneca
Everyone marvels at a square egg, but only the chicken understands the PAIN.