4. Stats to look for
Weapon DPS: The number one thing you need to do blood effective is a massive two-handed weapon, one with a slow swing time and high end damage. Dual wielding doesn't work for blood since you need the high weapon damage in order to fire off high damage Heart Strikes and Death Strikes.
Hit Rating to 8% Hit Chance: At 8% hit rating, all your two-handed damage and weapon strikes will hit. Since that's where most of blood's damage comes from, it's pretty much imperative that you hit that 8%. You'll preferably want to hit it all from gear (IE, around 263 hit rating), since that will transfer more combat stats to your pets. While hit rating past that 8% will still technically benefit you, since it will give a better chance to hit to spell-like effects like Death Coil, the benefit will be so low that it will be far better if you get other stats instead.
Strength: This is the mainstay DPS stat of the Death Knight. You'll get 2 attack power baseline from each point of strength, and there's multiple talents within the blood tree that provide an extra percentage of strength, which means every point of strength you get from gear is that much better. This is your number one DPS stat. Ignore stuff with raw attack power, you need strength. Also, all attack power you get will apply not only to your physical attacks, but will bolster your spell damage as well.
Armor Penetration: Since most of blood's damage is physical, armor penetration becomes a very valuable stat. It's a little hard to properly gauge how much armor penetration you'll need since it's a multiplicative stack with armor-reducing debuffs, but for the most part, more armor penetration never hurts. Of course, it's coming off of gear altogether in Cataclysm, so then it'll become really easy.
Expertise: As a blood death knight, you'll want 26 expertise. Some races (namely Orcs, Humans, and Dwarves) get an expertise bonus depending on what weapon they're wielding, but it might be best just to get 26 expertise from gear and talents if only so you don't have to worry about weapon swapping. 26 expertise makes your attacks impossible for boss mobs to dodge, which should be all you need, as you'll presumably be fighting behind the mob in groups and raids, thus nullifying the ability of your target to parry. On paper, strength and armor penetration are still more of a pure DPS boost point for point, but having an attack dodged can throw off your rotation and allow your diseases to drop before you've finished all your Heart Strikes, so it's nice to have them the table. Overall, you probably shouldn't worry too much about boosting expertise until you're at raid level content, and you can skip gemming for it, but once you're raiding, try your best to hit 26 expertise between gear and talents.
5. Stats of marginal benefit
While the following stats aren't completely worthless, you probably don't really need to go out of your way to look for them. These stats are listed more or less in order of effectiveness. Any stats I have not listed here, assume they are pretty much completely worthless for a blood death knight DPSer.
Critical Strike Rating: Critical strike is nice for a blood death knight in that it does help us proc Bloody Vengeance. That said, we have enough critical strike talents to take care of that for the most part. Strength and armor penetration will give you more bang for your buck.
Haste: Haste does give us a minor benefit in that it makes our strikes faster, but we really don't need it to any great extent, as we do have plenty of time to get our rotations finished within the base speed of our global cooldowns and diseases. Again, You'll get more bang for your buck from the stats in section 4.
Agility: Agility will at least give you some melee critical strike, but you really want critical strike rating for that, especially since raw critical strike rating will also apply to spell-like stuff like Death Coil. Leave the agility jewelry and capes to the rogues and hunters.
Attack Power: Sure, we do get some benefit from attack power. It does make us hit harder. But strength, again, gives you more bang for your buck, especially since strength can be directly buffed by such talents as Veteran of the Third War. Again, leave the attack power jewelry and capes to the hunters and rogues.
Armor: You probably want to wear plate armor just to get more use out of Bladed Armor, but don't go overboard with it either. The benefit is marginal compared to more straight up strength.
Stamina: Stamina certainly isn't a bad stat, it keeps you alive. But chances are getting straight up DPS gear will give you enough stamina to survive, at least at the raid and heroic dungeon levels. Let the stamina come to you, don't go to it.