This claim was made by a user on News Detective who argues dark chocolate is healthy due to its flavonoid content, being low in sugar and fat (compared to other chocolates), and also its mineral content. The claim references an article from the Cleveland Clinic (Link) who quotes Devan Peart, a registered dietitian and nutrition coach. Other credible sources, such as the National Institute of Health, support these claims, regarding dark chocolate as a “functional food due to its antidiabetic, anti-inflammatory, and anti-microbial properties” (Source 1)
On the contrary, a report from UC Davis cited popular brands of dark chocolate that were found containing heavy metals (Source 2). While this isn’t about the food item itself, it is indicating a risk to metals such as cadmium or lead that comes with the nature of processed foods. This is something less likely to occur in whole, organic foods which could be a different opinion held regarding “health”.
Overall, this is under the assumption that you consume dark chocolate in moderation as it would be unhealthy to over-consume anything. I find this claim more subjective rather than a fact. Yes, dark chocolate has undeniable benefits but like any food, there are always going to be potential downsides.