Dark Chocolate has many antioxidants, the antioxidants known as polyphenols and catechins.
The polyphenol families of procyanidins & epicatechins are strong antioxidants also found in many plants and vegetables including red grapes & wine. Proanthocyanidins play a role in the making of collagen & elastin and can lead to a stronger circulatory system. Proanthocyanidins are also shown to speed up recovery from plactic surgery. http://en.wikipedia.org/wiki/Proanthocyanidin
Catechins, in particular epicatechin found in dark chocolate, can reduce the risk of four of the major health problems: stroke, heart failure, cancer and diabetes.
Milk Chocolate has no antioxidant activity due to milk binding the antioxidants. To take advantage of the antioxidant activity of chocolate you need to have 70% cocoa solids and no milk.