Any chocolate containing sucrose is very unhealthy, and that's almost all the chocolate you're likely to encounter unless you specifically seek out sugar-free alternatives. Pure cocoa may be healthy. Cocoa as it's commonly prepared and served in candy and sweets is not.
It all depends on the context. If I just finished a massive weight lifting workout after a day of eating extremely low-carb, I'm going to need something to replace my glycogen stores. Even with that said, dark chocolate is still a high-fat food. According to the label the brand I'm eating right now contains 86% of my "recommended daily intake" of fat & only 6% of carbohydrates. Further, it provides one of the best fats for you -- the saturated fat Stearic acid.
Thus spake Eliezer:
It seems that many here might have outlandish ideas for ways of improving our lives. For instance, a recent post advocated installing really bright lights as a way to boost alertness and productivity. We should not adopt such hacks into our dogma until we're pretty sure they work; however, one way of knowing whether a crazy idea works is to try implementing it, and you may have more ideas than you're planning to implement.
So: please post all such lifehack ideas! Even if you haven't tried them, even if they seem unlikely to work. Post them separately, unless some other way would be more appropriate. If you've tried some idea and it hasn't worked, it would be useful to post that too.