When your Bloodwing strikes an enemy you gain health based on the amount of damage done (7% per rank).
Out For Blood is a Tier 2 skill in Mordecai's Rogueskill tree. It makes it so when your Bloodwing strikes an enemy and subsequently returns to you, you gain health based on the amount of damage done (7% per rank). This health seems to be lost if you are crippled before Bloodwing returns, even if you gain Second Wind before it does so(perhaps from an additional Bloodwing kill with Bird of Prey - this kill also will not give you any health with Out For Blood).
Additional levels may be acquired through the use of Class Mods