Blood Code are essentially jobs or class in other RPGs which can be obtained from your NPC companion by increasing your bond with them. Each of these Blood Code adds bonuses to your stats while decreasing others. Some can add an unique ability that others do not have. Each Blood Code has Gifts you can unlock, however some may require you to wear a the particular Code in question (unconfirmed).
There are three known Blood Codes so far - Fighter, Assassin, and Mage.
Fighter focuses on melee combat, increasing your endurance, strength and dexterity.
Assassin focuses on stealth, parrying and counter-attacks. Unlocks some unknown gifts and a special dodge ability. Unlockable by increasing your bond with Riki.
Mage focuses on offensive Gifts, increasing their damage and increasing your pool.
(Source: FeXtraLife's demo recording (YouTube) - link)