I recently leveled my DK's to 85. What i did was 55-68 as blood. I had my alts have the same talents spec as my main character. I only changed blood presence to frost presence on my alts, and had blood presence on my main.

On level 68 you get Howling Blast on the frost tree, and bought dual-spec and went frost. On questing i found Frost easier because it had more DPS. Once you go quest with frost x 4 make sure you change to unholy presence once you have it. It rocks! I still did dungeons as 4x blood, as it was easier tanking and pretty good dps.