A natural waxy, easy-to-rub-off coating on the skin
Bloom is much more obvious on some apples than others and can appear to have a color, such as purple. As a distinctive characteristic, it can help with identification if the apple has not been washed or polished.
The natural wax has a crystalline shape when seen under an electron microscope. Different species have their own characteristic crystalline shape. The crystals have a plate-lie appearance on apples but have a more protuberant shape on plums. The protuberances scatter light more effectively and make it appear that plums have more bloom than apples, but there is no relation between the amount of bloom, the shininess or dullness of the skin, and the amount of natural wax present.
The waxy coating makes the fruit surface repel water and also reduces the permeability of the skin. This can make it harder to get calcium sprays through the skin but also helps prevent moisture loss during storage.
Reference: For more information see the WSU Tree Fruit Research & Extension Center Postharvest Information Network.