People are giving you a lot of mixed information that is confusing.
Let me break it down better for you.
We’re getting the
final patch of Wrath as the default balance. This means that each of the 3 death knight Specs is technically capable of doing both tanking and DPS.
The reason why so many people are saying that blood is “the tank spec” is because of balancing.
Blizzard does this thing where, when they want to overhaul a class or spec in the next expansion, they will make nerf and buff certain aspects of the class in the final patch of the current expansion in certain ways.
In the case of Wrath, they knew that they were going to change the way Death Knights would work in Cata so they buffed the tanking capability of Blood to be vastly superior to the other specs while making those specs much better than Blood for DPS.
They did the same thing for Warlocks in WoD. They knew they were going to overhaul Demonology Warlock, so they purposefully made it suck in the final balance patch so “nobody” would play it and be upset when the next expansion came out.
So you are totally able to play as a tank as all three DK specs, but the final balance patch means that you’re gimping yourself by not playing Blood tank.