Archives of Opthamology Vol. 121 No. 7, July 2003
Eye black grease reduces glare and improves contrast sensitivity in conditions of sunlight exposure compared with the control and antiglare stickers in binocular testing.
MythBusters: Viewer Special II
Turns out, indirect light does more than make players blink. It creates "veiling glare," which reduces their ability to see clearly. Wearing eye black won't keep this optical phenomenon from happening, but it can tone it down. The stripes improve the eye's ability to differentiate between light and dark, and that increased contrast means you can see in greater detail. The better you can see the minutia around you, the better you can track an object as its speed increases — which is obviously important to outfielders following a pop fly's sunward trajectory.
I don't follow Opthamology literature, or know much about it, but see no reason to doubt the results of the study.