Translucent volatile image appears opaque
I create a TRANSLUCENT volatile image, render some stuff in it and then draw the image on to the screen. I expected that the rendering in the image would be "overlayed" over the existing screen graphics and that the regions of the image that I didn't render to would be transparent and not obscure any graphics on the screen.
But that is not the case. Instead I see an opaque black background which obliterates anything already on the screen. The graphics I rendered in the image are appearing but the image itself does not appear to be translucent.
So, is this the expected behaviour? Does a translucent volatile image start off life with an opaque black background? If so, how do I get it to be translucent?