What is the best way to store dark chocolate to preserve chocolate and flavour?
Chocolate should be stored in a cool, dry area whenever possible. A cold basement or wine cellar is perfect.
Moist environments, including the refrigerator, can cause the chocolate to bloom. This is when the cocoa butter separates and you start to see a white film at the top. It's actually still safe to eat this way and won't even affect the flavour much, it just looks a little odd.
That said, even tempered chocolate (i.e. baking chocolate squares) will melt at hotter room temperatures, so if the air is humid or you're in a hot climate, you'll typically want to refrigerate. Also, if you've just melted the chocolate to make a coating and haven't tempered it, then you must refrigerate it as it will melt even at colder room temperatures.
Otherwise, it's best not to refrigerate. It will generally keep for a year or more at 55-60° F (about 13-15° C).