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How do you store chocolate and why does it get white during storage?

Chocolate products should be stored in areas that are:

Free from odours; well ventilated; with a temperature of 18°C-20°C; at a relative humidity below 50%; away from walls and floors; out of direct sunlight.

Problems caused by poor storage include sugar bloom and fat bloom.

Sugar bloom makes the surface of the chocolate appear grey, covered in a thin layer of sticky syrup or covered with sugar crystals. It is caused by the dissolution of sugar from the chocolate by surface moisture, which eventually dries off leaving sugar crystals. One of its causes is storage in damp conditions. Chocolate will absorb moisture on the surface if stored in air at a relative humidity above 82% to 85% for dark chocolate or above 78% for milk chocolate. Impervious packaging will give protection, but overlapping wrapping will allow penetration of water at the folds or corners and the sugar bloom will appear near these points. It can also be caused by the removal of chocolate from cold storage without adequate wrapping protection. When chocolate has been stored below 10°C moisture will be deposited on the surface if it is brought out into normal atmosphere. When removing chocolate from cold storage it either needs to be transferred to a room with fairly dry air or needs to be covered until it has attained the same temperature as the outside air. Filled chocolates when contained in impervious packaging will, under warm storage conditions, produce a high humidity within the wrapping and this can cause diffusion of the syrup from the centre to the surface, which results in sugar bloom.

Fat bloom also makes the surface of the chocolate appear grey but it is made up of minute fat crystals. One of its causes is warm storage. The formation of fat bloom is related to the changes in structure of cocoa butter at different temperatures.

References:
B.W. Minifie, Chocolate cocoa and confectionery science and technology, 3rd edition. Van Nostrand Reinhold, 1989
L.R. Cook and E.H. Meursing Chocolate production and use. Harcourt Brace Johanovich, 1982