There is chocolate that is 100% chocolate bar. That would contain cocoa and cocoa butter.
Cocoa is actually really good for you, as is cocoa butter; it’s the sugar that makes it bad. There’s nothing innately in chocolate that would effect you negatively in the gym or otherwise.
If you’re going 100% chocolate, though, I would aim for quality chocolate - hand roasted. You want it without the chemical that neuters the taste of chocolate, so it has a more complex taste. You can also enjoy 80-90% dark chocolate this way as well, since it tastes far better than mass produced.
*I’m a bit of a chocolate snob.