Have you updated to beta 4? It might not work on that I haven't updated yet. On beta 1 I could just Add an icon without a name for it but it changed on beta 2 making you have to add a name to the icon you add to your home screen so I chose periods and blended it in with that white bar so it seems as if nothing is there. If your on beta 4 maybe it does not work anymore. The solution would be go back to a lower beta add the icons you want and then update, your icons that you added will still be on your home screen and they won't be black.
Well.... I'm on ios 6! ;-)
In any case, I'm looking forward to doing that - empty (blank) icons blended in the wallpaper colors. I'm assuming you are using ios7 beta 3, do you get colored icons which appear invisible on the wallpaper?
Also on ios6, it didn't ask to name it, so no need of a period ,but a black icon on a nice wallpaper makes going this route pointless!!! unless you have some trick
😎