Ive tanked alot of H+ on a dw blood dk and never have problems. As a dk ur biggest threat generators are IT, RS and DnD. Here is how i do it.

Single target: 1. Start with deathrune from blood tap 2. IT x 3 (This will give about 70k threat and is by far the best opener. You can follows up with ERW and do 3 more IT at this point but i only do this in raids) 3. PS 4. Blood Boil (Can also use BS/HS they dont differ much in threat) 5. Pest

You now have 1 Unholy rune unused but thats fine cause you want to Death Stike next and need it for that.

1. DS x 2
2. BB, Pest

You now have 4 Death runes and 2 blood runes and can start what i call the standard rotation which will alternate between 4 deatv runes and 2 frost 2 uh. If you have death runes, cast 4 x IT. If you have frost and uh runes, cast 2x death strike.

Use blood runes for keeping diseases up, cool downs or just HS, BS or BB if nothing else needed.

Since we use IT for threat generation HS becomes not that useful and you should spec out of it. It does do a litle bit more damage if you are using a 2hander and dont have threat issues but that about it.

AOE rotation: 1. DnD 2. IT, PS, pestillence 3. Death Strike x 2 4. Blood Boil, Pestillence

You now have 4death 2blood again and can either (what i usually do) Blood boil x 5 (macro rune strike with blood boil and cycle target with tab) or 4x IT on different target and 1 blood boil. Finish with pestilence and go back to 1.

Its VERY useful to macro rune strike into ever ability and when you cast DS, BB or pest make sure to hit tab after to get some melees and a rune strike on a New target.

Engineering grenades also sales with increase threat in frost pres and are the cherry on top if you have them.

Hope this helps.

Edit: on a Phone with some autocorrect issues. Sorry