Does chocolate spoil?

Chocolate contains a lot of fat known as cocoa butter. Normally when a chocolate bar is well made it's "tempered" meaning that it's been melted to a certain temperature that encourages it to solidify with a particular crystalline structure. It makes the chocolate "snap" when you break it but melt readily on the tongue. When a bar isn't stored properly, like in variable temperatures, the cocoa butter will migrate out of the bar to the surface and cause that white "bloom" that you see on the surface. It's not dangerous (not mold or anything) but will affect the texture of the bar, without all the cocoa butter it will be chalky. Most bars stored at a constant temperature between 50 degrees F and 75 degrees will keep for up to two years. Most manufacturers put an expiration date of 1 year to 18 months on their products. A chocolate bar left out of its wrapper will also oxidize, as you've mentioned, which means that any inherent moisture will leave the bar and the fats (cocoa butter) may become rancid.