What is it in dark chocolate that might be beneficial?

Cocoa contains compounds called flavanols which are a type of polyphenol. Polyphenols are naturally produced by plants and so are found in plant foods such as fruit, vegetables, tea, coffee, wine, grains and chocolate.

What research has been done on cocoa flavanols and health?

Research looking into the potential health benefits of cocoa flavanols covers a variety of different areas including effects on heart health, immunity, dental health, skin, gut bacteria and cancer. Whilst there have been some promising findings suggesting that cocoa flavanols can benefit markers of heart health (such as blood pressure and blood cholesterol), evidence relating to other health benefits is limited.

Source: Is dark chocolate really good for you? Q&A – British Nutrition Foundation