As a lvl 31 DRK, your mp restoration tools are limited to siphon strike and blood weapon. Granted, later on you'll get blood price, which recovers mp for every hit you take. For now though, if it's on trash, you'd probably be better off using the siphon strike combo. On bosses however, if your healer's up to it, you can drop tank stance for blood weapon on the boss, provided you can rotate cooldowns for it.