Vilian,
I accidentally made a black ribbonbar today.
I was working with the ribbonbar class trying to make a solid color ribbonbar. I commented out the gradient BMP brush that is made in the class to see the effect, and the ribbonbar is all black. So, it seems the black you are seeing is an effect due to something happening to the brush which is made from a bitmap.
I know that doesn't solve the problem but it is a bit more information about it.