This same thing is happening to me. I thought that I may have installed some malware or something like that but now I am not so sure. Also when I hold down my mouse to get the blue square on my desktop, it also turns the shape of the square black in such a way that you could draw with it. I really would like to fix this asap and another thing, it happened just after I got my second monitor. I don't know if that has anything to do with it, but it may help.
I think its something to do with deleting the original file in which you've made your wallpaper. I did this accidentally the other day and I've gone to the recycle bin, restored it and made it the wall paper again and I'm not getting the issue.
See if that works :)
I had a similar problem, and re-activating the background picture solved it - but it would re-surface again occasionally.
I then found out that this problem only occurred when I was using a certain 32 bit graphics program on my 64 bit system, it never occurred at any other time, so when I stopped using the offending program the black backed icons and black screen never returned!
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