Why can milk fat sometimes prevent chocolate bloom but sometimes cause it?

Answer: Its high-melting fractions inhibit bloom, while its low-melting fractions promote it

Its high-melting fractions inhibit bloom, while its low-melting fractions promote it ✓ — Correct. Milk fat mixes into cocoa butter and changes how it crystallizes, and which way that goes depends on the fraction. High-melting fractions stabilize the fat crystals and hold bloom back, while low-melting fractions can push crystallization toward the unstable forms that bloom. So the same ingredient can help or hurt depending on its melting profile.

It seals the surface so water cannot dissolve the sugar — This mixes up fat bloom with sugar bloom. Sugar bloom happens when moisture dissolves sugar and it recrystallizes on the surface, but milk fat works on the fat crystal structure inside the chocolate, not on keeping water out. Sealing off humidity and tuning fat crystals are two different storage problems.

It converts the stable Form VI crystals back into Form V — This overstates the mechanism. Bloom often involves Form V slowly turning into the more stable Form VI, but milk fat does not reverse that. What it does is shift crystallization, and depending on the fraction it can either hold bloom back or speed it up.