Compared to lighter varieties, does dark beer have antioxidants in higher amounts? Yes, it does. In fact, these test results for Guinness stout produce an ORAC value that is over twice as much as Bud Light. The reason? Because many of the antioxidants come from flavonoids, which are formed when the malted grains used to make them are roasted for greater amounts of time.
Sadly though, the whole picture must be looked at here. While its true that dark beers have more, it's on a relative basis. Even though this stout has a value of 210, it's really almost nothing. It's more comparable to a McDonald's fried chicken sandwich (coming in at 180), than it is to plant-based foods such as red leaf lettuce (coming in at 2,426). So sorry, downing a stein of booze is no excuse to skip your greens!
Research Support, Non-U.S. Gov't: Carlsen MH, Halvorsen BL, Holte K, et al. Nutrition Journal NIH Jan 2010