Black Diamond [Pre-formation] under White Diamond’s control...
Black Diamond stood as a sentry for the first century or so of his life. As time went on, he was called for higher-caliber jobs (such as assisting Yellow and Blue Diamond in conquering planets). The mask he wears suppresses his personhood, but only so long as he is given instruction. That is the flaw of the mask, and White knows this.
Should White Diamond forget to tell him to “Stand by” after he has completed a task, her hold on him is slackened, even if only for a moment. But in the background of his primary directive, he is aware, and he is intelligent and patient enough to wait for the right moment to come.