Alt Tab black screen with full screen

what fullscreen settings are you using? There's "change resolution mode", "stretch to fill window" and Ultimate Fullscreen extension (windowed fullscreen). Also, how do you want your app to behave when the user is Alt tabbing? should it minimize, move to background, resize window, etc.? Generally I think Alt tabbing is only possible with windowed fullscreen solutions.

... and an example .mfa would also be helpful

That's very weird, hmmm did you tell the window to check if it's minimised and then resetting it to full screen and exit the event and if it's full screen already to minimise the game window and then exit the event?

Ah, now I understand what you mean. The app's preview window shows black instead of the last rendered screen when Alt-tabbing out of fullscreen mode for the second time. I can reproduce it on my system (Win 8.1). I think this might be a minor bug related to how Fusion handles windowed fullscreen itself. I use Ultimate fullscreen extension and this bug did not occure so far, you could try to implement it and see how it goes. Anyway I personally think this is a minor issue, as your app displays properly once the window is active again. I doubt any user would flame you for leaving it like this

Try getting the values of screen width and screen hieght and then change the resolution of the window width to screen width and the height of the window to screen height