By definition, there can be no milk in dark chocolate. For this recipe, choose a mildly sweet chocolate. If the label states cocoa solids (also called chocolate liquor), select one that contains at least 35%. However, the higher the percentage of cocoa solids, the more bitter the chocolate will be.
The fruit pieces need to be dry when dipping them into the fondue. If they're slightly wet, the chocolate might not stick as well.