Definition: dissolving cocoa powder in a hot liquid to deepen its flavour before it goes into the batter.
“Blooming” cocoa is a small step with a big payoff. Whisking cocoa into something hot — coffee, boiling water or warm milk — wakes up and disperses the flavour compounds, so the finished bake tastes richer and more chocolatey rather than dusty. It’s why so many good chocolate cake recipes ask you to add a hot liquid at the end and warn that the batter will look alarmingly thin. That thinness is correct; the reward is a moist crumb and deeper flavour.
In practice: add the hot liquid last and don’t panic at the runny batter — it bakes up beautifully.
Related:cocoa powder,crumb.