A stroke happens when a blood vessel bursts in the brain. The free-radicals from blood and the lack of oxygen causes brain cell death. The brain can repair the damage by rerouting the signals going to the effected brain cells to other brain cells. At least in the case of this study, the brain cells started doing double duty instead of changing jobs. Further study has to be done to confirm this.
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.