Press and hold something on your keyboard, like the mouse button to mine or something else. Now press the fullscreen key. While you get the blackscreen stop holding the key/mouse button. Now you're able to switch over to another window by pressing alt+tab(Microsoft Windows), your minecraft client thinks now your holding the key/mouse button.
Here a tipp/advice: switch over to two another windows. So if you minimize one window you wouldn't get thrown back to minecraft.