Apart from caffeine, chocolate also contains a substance called theobromine. It is a stimulant that has a similar effect to caffeine and is linked to causing gas problems in babies. White chocolate contains lower amount of theobromine and may have fewer side effects on your baby. I would say that you could eat it but in moderation. If you eat a small piece of dark chocolate, reduce your caffeine intake for that day. Also observe if your baby reacts to it. If it leads to gas in your baby, then perhaps you will need to hold off dark chocolate for now and opt for milk or white chocolate. For more useful information: http://www.newhealthadvisor.com/chocolate-and-breastfeeding.html
Yeah from personal experience, chocolate is a total no no for my bub - she ends up staying up all night due to gas. :(