In this post on the Nikon Speedlight SB-400, I noticed black objects looked reddish.
When looking through a review of the iPhone 5's camera (http://www.dpreview.com/articles/6867454450/quick-review-apple-iphone-5-camera/3 - in case the link is outdated, search for m8 infrared black), they were discussing possible causes for the purple fringing problem. One of the (rejected) suggestions was infrared light, which makes black objects appear purple! Apparently, the Leica M8 has that problem, and it looks exactly like what's happening here. The fact that this only happens under sunlight (which has lots of IR) and not fluorescent (hardly any IR) is more evidence.
But I thought UV filters block both UV and IR. Or do only specific ones do that? Does this mean I can take IR pictures with my D50?