I have two solutions that you can utilize, but they come with their own pitfalls.
To get rid of the black bar, you can do call BlzFrameSetAllPoints(BlzGetOriginFrame(ORIGIN_FRAME_WORLD_FRAME, 0), BlzGetOriginFrame(ORIGIN_FRAME_GAME_UI, 0))
in custom script. However, this will remove the black bar underneath other parts of the UI as well.
Another solution would be to create a transparent image and set its path in the import manager to Textures\Black32.blp since that is the texture path for the black bar. However, anything that uses the same texture would also be affected. I reckon that this texture is pretty common, so changing it could very likely affect the visuals of several UI components. This solution will also have the effect of removing the black bars entirely, but it does not come with clicking issues from extending the World frame.
On 1.31, there is not much you can do about the black bars on this patch.