The Blood tree often gets a bad rap among those not in the know. It's a decent leveling tree, but it's generally considered not to have the tanking power or AoE DPS Potential of Unholy or Frost. By all indications, that has changed in 3.1, especially in the latest patch. Not only have many key Blood talents and skills received key buffs or changes, but the very nature of Armor Penetration has changed, with the 25% benefit buff making Blood's mostly physical damage dealing style well suited to take advantage of the change. This is quite a shift from just before 3.1, when many were afraid disease-less Blood would be wiped out and leave the tree with no viable hassle-free DPS specs.
Blood DPS can now safely use Death Strike as their standard Frost/Unholy Rune dump thanks to the buff baseline buffs and the new Improved Death Strike talent, which in alone solves a lot of Blood DPS problems, since you're no longer locked into 13 points of Frost for Annihilation. This allows heading into the Unholy Tree, where one can pick up major useful talents such as Morbidity, Epidemic, and Master of Ghouls, resulting a build that might look a little like this. This build benefits from grabbing extra strength, Death Coil damage, disease duration, and spell hit in Unholy, which would be near impossible if Blood still had to grab Annihilation. Since Death Coil will be your main RP dump and you'll be shooting off free Death Coils due to sudden doom, you'll want the damage and the extra chance for them to hit.
Some people have been a bit concerned that the Tier 8 4-piece bonus doesn't cover Death Strike, but don't expect Blood to fall behind too much because of that, because it does apply to Heart Strikes. That's really the secret of the build. Death Strikes are just there to generate Death Runes for Heart Strikes -- or, if you're AEing large groups, Blood Boil.
The two major disadvantages kept by Blood DPS are needing to babysit the Ghoul (since you won't be able to afford Night of the Dead) and the tightness of their rotation, needing to spend as many runes as possible on Heart Strike before Frost Fever and Blood Plague fall off. The former problem - is the perma-ghoul worth it for Blood DPS when it will die so easily - is probably one that will need some further exploration, and the latter shouldn't be as much of a problem thanks to a few points spent in Epidemic and Death Strike not removing diseases like Obliterate did without Annihilation. Overall, if Blood goes live looking close to how it does now, it should be a very tempting tree for DPS.
One of Blood Tanking's major weaknesses has always been that it's the weakest AE tree. That is pretty much gone as a problem is 3.1. The new disease-less Blood Boil is actually working remarkably well as a snap AE aggro tool, enough so that you shouldn't really need to worry about heading 21 points into Unholy to grab Unholy Blight. A Tank build like this should work pretty well both for survivability and threat. If you have a Frost DPSer in your raid who can cast Improved Icy Touch, all the better, you just freed up 3 points.
Also nice for Blood Tanks is the new buff to Scent of Blood. The latest iteration gives 10 runic power per a charge and can be activated by dodging, parrying, or simply by taking damage. Preliminary tests on the PTR are placing it at almost always up in multi-mob situations, and up enough to provide 2-3 runic power per second on average on single target fights.
This should make it a lot easier for Blood Death Knights to keep chaining Rune Strike and cement their place as solid single-target aggro holders. Of course, Scent of Blood is also conveniently low enough in the tree that Frost and Unholy tanks can take it, but Blood will probably still find it the easiest to take, as they'll probably have 3 spare points on the way to getting Will of the Necropolis and Heart Strike easily.
Speaking of Heart Strike...
There are some Death Knights who are asking for a new secondary effective to Heart Strike now that Blood Boil is an effective AE damage and threat tool. There's certainly a good argument for the cleave being less useful than it once was, but it's also useful to keep in mind that it wouldn't be the first time Blizzard has left a strike without a useful secondary ability, such as when they removed the HoT removal from Plague Strike.
Overall, even with that less useful secondary mechanic, Heart Strike is still a very fine damage dealer. Blood is a tree that's currently headed into 3.1 stronger than ever, and if you haven't taken a look at it lately, you might want to strongly consider a second one now. I'm probably going to keep on with Unholy, but at the same time, all that health recovery and a Dancing Rune Weapon churning out Blood Boils is just looking way too juicy for words..