Chocolate bloom explained

The stable crystal form that tempering produced converts slowly to an even more stable form, and as it does, fat migrates to the surface and recrystallises there as a pale grey film or streaking. Warmth accelerates it, and repeated temperature cycling accelerates it more than a steady warm temperature does. Badly tempered chocolate blooms within days; well-tempered chocolate stored properly takes many months.

A different mechanism that looks similar. Moisture condenses on a cold chocolate surface — typically when a bar comes out of the fridge — and dissolves surface sugar. When the water evaporates the sugar recrystallises as a gritty white film. Sugar bloom feels rough to the touch where fat bloom feels smooth and slightly greasy.

Chocolate has very low water activity, which is why it keeps for months without refrigeration and why mould is essentially not a risk in a plain bar. The white film is the chocolate's own components rearranged. It can be eaten, melted, baked with or used in ganache with no problem at all.

Temper correctly, and store at a steady 15–18°C in low humidity, wrapped, away from strong smells and out of the fridge. Steady temperature matters more than the exact number. If you must refrigerate in a hot climate, wrap the chocolate airtight and let it come fully to room temperature before unwrapping, so condensation forms on the wrapper rather than on the chocolate.