The bloodbind is a canta malus (darksong) songspell. It is a powerful spell in which blood from different mages is combined to form an unbreakable bond and allow them to share abilities. The six mermaids summoned by the Iele (besides Astrid) perform this spell before escaping the Iele's Cave. Astrid later completes the bloodbind, despite the fact that she cannot sing.
Later, they discover that they can speak a range of languages, perform better songspells and control their emotions. It is also mentioned in Sea Spell that if a member of the bloodbind formed by the daughters of the seas died, then the others would be able to sense that the connection had been severed.