There are 2 types of chocolate bloom – cocoa bloom and sugar bloom.
Cocoa Bloom can occur if a chocolate has been subject to heat of 26 degrees or more. Even though the chocolate may only have gone slightly soft, one or more of the fats in the cocoa butter can melt at this low temperature, changing the chemical structure of the chocolate. This cocoa butter then sits on the outside of the chocolate, often as small white dots, and can actually be wiped off by your finger. Many people think it is mould but chocolate itself does not ever go mouldy unless it has been kept for a while in a fridge and subject to moisture. The chocolate is still edible and once melted in the mouth tastes the same, but may have lost visual appeal and good snap. Don’t throw it out though – melt it down and use it in a cake or ganache if you don’t know how to re-temper it.
Sugar Bloom is what happens when chocolate is too cold or is in too humid an environment. Moisture will form on the outside of the chocolate which attracts the sugar to come out of the chocolate and crystalise on the outside. This may look similar to cocoa bloom or may just give the chocolate a very rough look and feel (you can sometimes feel the sugar on the outside). Again, the chocolate is still edible and will taste the same once in your mouth.