So I have a basic knowledge of dk tanking now. I noticed blood rites allows death strike and obliterate to turn into death runes . I know to use death strike to heal and after 80 that creates an absorb shield but is there a time during low damage or no damage that obliterate is more usefull for damage /dps /threat? Also should i be trying to use festering strike to keep diseases up on bosses or is it simplier to just reapply separate so you dont use the blood rune and screw with rotation /priority ?
Don't use Obliterate, you're not dps'ing. You should be using Rune Strike when you have frost/unholy runes on complete cooldown. Rune management is one of the biggest deals about being a dk. (http://www.wowhead.com/spell=49501) Blade barrier is something you should be keeping up on yourself, and arguably the best way to do that is to either Heart Strike or Blood Boil (depending on the situation) within two seconds or so from the completion of the other blood rune's cooldown. This keeps both on active cooldown enough to keep blade barrier up, but doesn't allow for (http://www.wowhead.com/spell=81229) Runic Empowerment to choose a blood rune. That's very important because it leaves only Frost/Unholy runes able to be refreshed, therefore more chances at death strikes. As far as your diseases, use a frost for icy touch and an unholy for Plague Strike right up front. When those are about to wear off, use outbreak to keep them up. Beyond that, I recommend using (http://www.wowhead.com/spell=45529) Blood Tap before breaking a frost/unholy pair for a disease when you could use it on a Death Strike. It's alright at the beginning of a fight where your Blood Shield and heal from Death Strike wouldn't be that big anyway, but later on, one more Death Strike could save your life.
This is only the tip of the iceberg when it comes to DK tanking though, and I won't say I'm anywhere close to the best at it either.
Festering strike is for unholy DKs. Obliterate is for Frost DKs. Death Strike is for Blood DKs. Those aren't absolute rules, but from a blood DK tank's point of view it pretty much is. You should never use FS/Obli as a blood DK.
Keeping diseases on your target, you've got 3 options:
1) Don't. If you are tanking weak opponents for very short periods, just don't put diseases on them. Alternatively, if you are in a raid group with more than 1 tank (or other classes that can debuff for you). In most cases, you will at least use the next:
2) Outbreak. Once per minute (until patch 4.3) you can put up both diseases. It can miss though.
3) Icy touch, Plague strike. This is a last resort. If you are tanking something that hits hard though, you need to keep the attack speed slow (frost fever or equivalent) and physical damage reduction (scarlet fever) debuffs on your target. If no-one else is doing it for you, you will have to sacrifice some runes on putting up diseases by hand.