The grey streaks that sometimes appear on chocolate are caused by two different factors:
fat bloom and sugar bloom.
Fat bloom occurs when cocoa butter, which is a polymorphic fat (which means it can take various forms), crystallises. Cocoa butter can crystallise in six different ways, and each type of crystal has its own properties. If the cocoa butter is crystallised under an unstable form, it will tend to recrystallize into a more stable form. This recrystallization produces the thin white layer which forms at the chocolate’s surface. The main triggers of fat bloom are poor tempering, incorrect cooling of the chocolate, temperature fluctuations (during transportation, distribution and storage) and mixing the cocoa butter with incompatible fats.
Sugar bloom is caused by the formation of large sugar crystals on the chocolate’s surface and is caused by moisture exposure. It can also be caused by condensation on the chocolate’s surface or by storing the product in damp conditions.
To find out which of the two different phenomena has occurred, try touching the chocolate: the surface of chocolate with fat bloom will melt easily and be sticky and smooth. On the other hand, the surface of chocolate with sugar bloom will feel dry and coarse.
Fat bloom changes the look and appeal of the chocolate, but does not change its taste. You can actually easily remelt a plain bar and obtain nice looking chocolate again.