Why do I get this rendering problem when I enable blending?

I want to render a cube on a transparent background. I did enable blending for this and tryed to set alpha 0 for background. The window background is transparent. If the window background would be black (the black color is not from the windows background, I am not sure why it’s rendering it) , the cube would render normally. When I move around it acts like drawing with a white brush. Any help, please ?