My guild and I hardly ever PvP, just started ToC 10 last night, but got stuck on Faction Champs. Part of the problem was that my PVE tank and dps specs weren't very helpful, very little cc and not enough damage to compensate. I was pretty useless.
The most useful part about DKs for 10man Regular ToC (Especially since you're just getting into it), is successfully kiting around a melee. Generally, if you know what you're doing, alternating between death grips, chains of ice, and dark command you don't need to respec at all. Just try to pull the melee class you're assigned to to the back of the room. If you really want to have an all "useful" spec for CCing on 10man ToC, Even something really useful on Heroic which is a lot harder the spec I posted below is pretty good. Pretty much you have all the useful spells, AMZ, Ghoul, Death Grip CD, Chillblains Slow, Increased Chain Range, Hungering Cold etc, Your DPS will probably be garbage, but as said before your focus is on kiting the mobs! Since you said your Tank/DPS Spec weren't very helpful, you should certainly either practice using Death Grip, Dark Command and Chains of Ice or just respeccing to what I posted below!
If it s normal 10m you dont need a specified spec for locking a melee. As the person above said, 2 taunts and COI do the trick just fine (to be honest in 10 m you dont really need to lock down a melee but i assume this also depends on your raid)
I was once though having as offspec the classical 0/17/54 UH PVP build and indeed it was easyer to CC due to increased COI range, decreased CD on DG, Desecration and ghoul stun but i believe it s a little bit overkill even for 25m hard mode (unless you maybe go for immortality run or something and need the extra safety )
Have always done this in my standard Blood tank spec and gear (human racial means I dont need a pvp trinket).
My job is usually trying to lock down either Rogue or Warrior, though been on Shaman and DK a few times.
As said above in Normal its quite easy, but in HC you cant taunt, but COI and DG works just fine... its a matter of seing who is the target and use DG if its a clothie, and otherwise keep COI on your target so healers etc. can run away easily - forget DPS, its just not your job in this fight.
Control and making it easier for others to get away from damage doers are the way to win this fight... (though locking down/killing healers should have a priority too).