Dark chocolate is gaining popularity these days because of its antioxidant content. The darker(more cocoa solids) the more flavonoids it contains.
A research was done comparing the blood pressure levels of dark chocolate eaters and white chocolate eaters. After several weeks, the blood pressure of dark chocolate eaters dropped while the latter maintained its blood pressure.
That's good news as far as blood pressure is concerned but the research was made on a daily intake of a chocolate portion containing only 30 calories. It's roughly one small square of chocolate from a bar. That's the safe amount to bring out the benefits of dark chocolate as far as antioxidants are concerned. Larger servings would elevate the intake of sugars, calories and fats which may not be beneficial for the health.
When choosing a dark chocolate, opt for the one containing at least 70% cocoa solids for it contains more antioxidants. Limit your intake though because the antioxidant benefits may be offset by the effects of high sugar, calorie and fats that chocolate also contains.