Strength in the blood
Blood's PVE role is also its PVP strength: A blood death knight is damned hard to kill. Of course, generally in PVP being hard to kill is great, but you need to be able to kill people too. Luckily (in Battlegrounds, especially), there are things you can do to leverage the survivability of blood without worrying as much about whether you personally are killing someone.
Battlegrounds with PVE components, such as Alterac Valley and the Isle of Conquest, are one place where a blood death knight can shine. the NPC generals and captains in these Battlegrounds aren't necessarily raid-boss level in tactics, but they still hit hard. If you, as a blood death knight, tank them just like you would in PVE, you will be a hero for sure. There've certainly been AV weekends when I have, for lack of any alts to level, queued on my death knight as blood, just to tank the bosses. Don't worry too much if you're paranoid about enemy players coming in and dive bombing the raid; you can tank in PVP gear if you think that'll be an issue.
Flag-running Battlegrounds like Warsong Gulch and Twin Peaks can also be a good place for a blood death knight to shine. This is a little more up to debate, though. Other classes such as druids, mages, and rogues are going to have more ways to shed debuffs and snares and speed buffs to pull ahead of the pack when flag carrying, so they are, in theory, possibly even a better choice than blood death knights for flag carrying. That said, if you have a backup healer or two, your advantage is going to be that you cannot die. Got half the enemy team beating on you? No problem. Vampiric Blood, Icebound Fortitude, and your other defensive cooldowns can take care of that well enough, especially if you can get off a few Death Strikes on your pursuers as well.
Base defense battlegrounds such as a Arathi Basin and Battle For Gilneas do pose a little more of a problem for death knights since, mostly you need to kill the other side before they can capture your bases or kill them so you can capture their bases. That said, a blood death knight can still find a decent niche here, too. Since these Battlegrounds generally require you to click on the flag and channel a bit before you can capture it, all you need to do is stay alive and keep damaging the enemy while you alert the rest of your team. You probably won't be able to take down those two or three Horde by yourself, but if you can use your cooldowns and Death Strike to stay alive until reinforcements come, you can still save the day. Of course, if you're in a PUG, there's no guarantee that anyone will come -- but hey, it's still a pretty huge rush to hold off multiple enemies longer than anyone should have a right to. Eat your heart out, Spartans.
Compensating for weaknesses
Of course, even if you're tank-specced, don't think you're out of options for offense. Even in their base state, Chains of Ice, Death Grip, Mind Freeze, and Strangulate, among others, are amazing control tools even in their base forms. On the flip side, though, Necrotic Strike is going to be a lot harder to use. The easiest place to use it is after your Death Strike has converted your non-blood runes to death runes, but then you'll be giving up on the self-heal and Blood Shield from Death Strike. It's definitely a hurdle to overcome, but you'll still want to hone your offensive skills.
On the gear side, you'll still want PVP gear, of course, and the PVP gearing tips we talked about last week still apply to blood. Resilience is king, and resilience will protect you even better than tank gear, especially against casters, who can completely ignore all that parry and dodge rating. For your secondary stat, focus on mastery. Survivability is the best reason to play blood PVP, and mastery remains our best survivability stat, especially in PVP.
The talented blood PVPer
If you're PVPing blood, I'm generally going to assume you're using your PVE spec. That's definitely fine, of course. There's enough solid power in a PVE tank spec to get you going. That said, if you want to do some tweaking of your spec for PVP purposes, here are a few suggestions. Frost is likely going to be your better secondary tree, since you can pick up Lichborne, Endless Winter, and Icy Reach, which are all incredibly nice PVP talents. Of course, your PVE spec may have already grabbed these for the PVE benefits as well. You may also wish to save two talent points for Unholy Command over in the unholy tree.
In the blood tree, you may want to consider picking up Hand of Doom, allowing you the ability to keep casters locked down slightly more often. The other thing you can do is reglyph a bit for PVP. There's unfortunately not many specifically PVP-focused blood glyphs, but you may want to find some room for Glyph of Strangulate, at least, and you may find that Glyph of Death Grip helps you wrangle in fleeing flag carriers or healers trying to dodge behind pillars that much easier. Glyph of Rune Tap can endear you to a party if you run with a pack of PVPers.
Learn the ropes of endgame play with WoW Insider's DK 101 guide. Make yourself invaluable to your raid group with Mind Freeze and other interrupts, gear up with pre-heroic DPS gear or pre-heroic tank gear, and plot your path to tier 11/valor point DPS gear.