Here in Europe the Apple TV's "Aerial" screensaver hasn't worked for two days now. I just get a blank screen. If I'm not mistaken, this could be a server-end issue at Apple as I don't believe the screensavers are stored on the Apple TV itself?
The Apple screensaver are downloaded and stored locally. I have had a 'blank' show up every now and then. It is probably a software bug that happens when goes to play one but it doesn't load properly. As soon as it cycles to the next it will display the next.
I have found the aerial screensaver to be a little buggy. Most commonly, the screensaver just doesn't start when it should, with the screen dimming and then coming back to normal brightness. I usually fix this by restarting the aTV. A couple days ago, the screen just went totally black. Fixed that by going into settings, selecting a different screensaver, then going reselecting the Aerial one.