Different players hold different opinions about Death Knight specs. No matter what happen, there are some pinpoints to talk about. I am level 80 Death Knight, I want to share my knowledge through my own experience. Let us get down to this as follows.
Blood is poor on the AoE threat compared to the other trees. It is poor on the mitigation versus the other trees. Blood is a superior DPS spec. It is also a great tank spec to receive additional healing, mitigation is better than boosted healing received because no amount of healing can make up for an unlucky streak of damage, but mitigation might prevent it.
Frost has the most mitigation and passive avoidance available. It also extends the duration on your "oh shit" button. It is competitive on the AoE damage but only on the "active" AoE. However, Howling Blast provides for a giant amount of AoE burst threat, so that AoE tanking is not an issue.
Unholy has more mitigation than Blood, less mitigation than frost. Unholy is the AoE threat monster, with many passive AoE threat abilities, it makes for an excellent AoE tank. Unholy DK's provide for better AoE threat than any other class/spec available right now.
For example, Protection Warriors have an "activate once, slight cooldown" AoE ability. Druids have a frontal-only AoE that forces them to slack on the single target threat. Paladins have Holy Shield and Consecrate, but Unholy DK's win out. Not only do they have a consecrate but they have two. They have a Death and Decay that can be placed like a consecrate anywhere within range and a second consecrate (Unholy Blight) that roams with them, whereas a pally's consecrate stays in place. To combine that with "Wandering Plague" which allows a DK to do additional damage to targets while doing nothing, and you've got an AoE threat monster.
But most of the time, I would spec Unholy if I were tanking heroics or other 5-man content. As usual, stuff does not hit that hard and wiping is a result of a dead healer. The Area of Effect treat provided by the unholy tree ensures that you don't have an accidental NPC ganking a healer.
If you're main tanking a 10 or 25 man instance, I'd go for frost. As a main tank, you want concentration on mitigation and single target threat, which Frost affords very well. More often than not, you can still AoE tank if such a thing is necessary but it is better handled by the off-tanks while you take a beating from the nastier guys. Dual wield is not preferred until your expertise is up higher. If you're lacking on the hit and expertise, then you really should stay away from dual wielding so you don't parry-gib yourself. With raid buffs and having a high parry rate yourself, you could be swinging 2 or even 3 attacks a second. Dual wield could be beneficial down the road once you're expertise capped, as it can generally offer you more white-damage threat and spells like Howling Blast aren't based upon weapon damage. Dual wielding will also allow you to pick up tanking stats on your weapon, whereas two-hander has none.
If you're off-tanking 10 or 25-man content, I'd go for Unholy. Most off-tanks are really just holding threat on stuff that won't hit hard, but need to be kept off of AoE DPS and healers. Each tank will have their assignments, but often times, something might get away, or just not get picked up, or in chain-pulls, there's not enough time to sort out every single mob. Being an unholy off-tank ensures minimal wipes on trash.
As for spec you really want one drafted up I can pull one up for you or you can find some other specs but I will let you know that very few specs are set in stone as far as the Death Knight community is concerned. There are many cookie cutter specs, but the "best" is far from being decided upon.
Still Blood is a subpar tanking spec. Not that tanking can't be done, but the "mitigation through healing" spec isn't optimal in a tanking environment and blood is extremely weak when compared to Frost and Unholy for AoE threat. You will find yourself struggling more than you really need to while tanking.