LogicBricks has a "ScreenFade" brick that can be used to achieve fade-to-black or similar effects for scene transitions or waiting on something to load.Thank you for your prompt reply, that's too sad. However, the mentioned feature could indeed make scene creation visually more interesting. I once tried to achieve a similar effect by adding a transparent mask image in front of the window, but it would become misaligned when the FOV changed.
If I remember correctly that works by literally putting a black cube around the camera that is just rendered after everything else. You could achieve black bars at the top/bottom in a similar way. Much easier than going via post-effects.