transform RGBA to RGB after blending

I want to achieve a fairly simple operation in VTK. My starting point are several 2D images (same dimensions) that I want blend together over a black background. All images have alpha channels and some (semi)transparent areas. After blending, I want to remove the alpha channel to get an image without transparencies (i.e. a pixel that would have been fully transparent in all source images would would result in an opaque black pixel).
After googling a lot I still have no idea how to achieve that. Any advice is welcome. Thanks in advance.