Chocolate bloom
Bloom is the whitish film or streaking that appears on the surface of chocolate. The product stays safe to eat but loses its appearance, gloss and often its texture. There are two types with different causes.
Fat bloom happens when cocoa butter migrates to the surface and recrystallises there into the stable but unwanted form VI. The causes are faulty tempering, storage at fluctuating temperatures, or contact with a fatty filling such as nut paste. It looks like a dull greyish film, sometimes like marbling.
Sugar bloom has a different origin: moisture condenses on the surface, dissolves the sugar, then evaporates and leaves crystals behind. The classic scenario is chocolate taken from the fridge into a warm room. The surface feels gritty, whereas fat bloom feels soft.
The prevention is the same for both: correct tempering and steady storage at 16–18 °C somewhere dry, without sharp swings.