All chocolate should be kept in its original wrapper if possible, or wrapped in foil and then plastic. Store in a cool, dry place at about 18°C or 65°F. Do not store chocolate in the refrigerator. High humidity will shorten the shelf life of chocolate.
Unsweetened and dark chocolates will keep for years in ideal conditions. Milk chocolate will keep for up to a year, and white chocolate, because of its high butterfat content, will keep for 6–8 months.
Certain chocolates, such as couverture and bittersweet chocolate, as well as prepared chocolates such as truffles, freeze well.
Incorrectly stored chocolate will ´bloom´. ´Sugar bloom´ occurs when chocolate is exposed to moisture. As the moisture evaporates, it draws out tiny particles of sugar, leaving the surface looking ´mouldy´.
´Fat bloom´ occurs when the chocolate has become too warm. The cocoa butter begins to melt and forms grayish-white areas on the surface. Although unattractive, bloom does not affect the flavour and the chocolate can still be used.
(Source : Irresistible Chocolate by Elizabeth Wolf-Cohen)