For me (after running multiple DK teams) it has shown that pre-70 blood is ideal as the scaling is just suck and Death Strike can outcrit Obliterate big time at that time, together with Heart Strike cleaving it's sick for both instances and leveling. At 70 and higher you should buy dual spec (it's cheap anyway) and make all of them frost to just make 1 spec back to blood for instances to make the healing easier. Frost generally started to do better for me at that point, both PvE & PvP.
If you want to do some BGs in between leveling frost is just rape compared to blood as people can't kite you at all and the Lichborn anti-fear + self healing wins.