A white chocolate bar develops greasy gray-white streaks after warming and cooling, but its smell has not changed; what most likely happened?

Answer: Cocoa butter reformed

Surface sugar dried — Sugar bloom is a real chocolate problem, so this is a plausible trap. But it is usually tied to moisture or condensation and tends to feel dry or grainy, not greasy. The stem points to warming and cooling with an oily-looking surface, which fits fat bloom better. That contrast teaches a useful kitchen diagnostic: rough and dry suggests sugar; smooth and greasy suggests fat.

Milk proteins clumped — Milk proteins can change in cooked dairy foods, but a finished chocolate bar is not usually forming gray streaks because its milk proteins clumped. The visible streaks are better explained by the fat phase moving and recrystallizing at the surface. This is why bloom is discussed alongside tempering and storage temperature, not like curdled milk. The white color can make dairy seem guilty, but the culprit is usually the cocoa butter.

Cocoa butter reformed ✓ — Right. In fat bloom, cocoa butter partially melts, separates or migrates, and recrystallizes into a gray-white surface layer. Puratos describes unstable cocoa-butter crystals recrystallizing into a thin white layer, and Food & Wine gives the same heat-change mechanism. The counterintuitive bit is that this is the same fat physics that makes chocolate glossy and snappy when controlled. Bad storage turns a feature into a blemish.