How to use blood magic effectively in battles?

[Taqwus]

Hell Power, if you've got multiple other paths.

In general, while Blood got a combat boost over D2, it's still a summoning-heavy school -- especially if you're Lanka or Mictlan with all those tasty national summons.

Edit --
Oh, and certain tasty items, as well. Blood Thorn is the only 1HD non-unique life-draining weapon in the game, IIRC. Hell Sword is 2HD, but gives fire resistance, 'zerk, AND lifedrain. Armor of Souls is nice if you're facing lots of astral mages or mindblasters, for the heavy MR boost. Boots of Youth, for, well, youth. Contracts give devils...