Best spec ever for leveling is UNHOLY! why ? travel time baby, you get On a pale Horse talent that gives each DK "crusader aura". Also perma pets and mass ae dps! add a healer and no need for blood. Unholy is also a lot of eye candy. more fun than the other specs IMO!

If you plan on doing instances with them BEFORE 70, and without a healer.. than,yea, blood. If you want to level to 80 first, Unholy will not let you down!