Wait, dark chocolate is sour? Very high concentration (80%+) dark chocolate is close to powdery and has some fruity notes, but it's usually slightly bitter rather than sour to me. It's not my favourite but I can't think of it as being sour.
There are some chocolates that are definitely sour, sometimes that's a desired flavor when making chocolate. For example, the last batch of single origin Peru chocolate was noticable more sour than the Bolivian and Ecuadorian ones I made. I'd actually intentionally selected my beans knowing this.