Blue Diamond is one of the characters in Steven Universe. She is one of the main villains, along with Yellow Diamond and (supposedly) White Diamond.
She is one of the Gem Matriarchs on Gem Homeworld and a member of the Great Diamond Authority ruling over Homeworld. Blue Diamond was apparently responsible for overseeing an attempted colonization of Earth, and later, the attempts to quell the rebellion of the Crystal Gems.
She is voiced by Irish singer Lisa Hannigan.