Without releasing the drag, click the middle mouse button.
If the platform's normal behaviour is to cancel a drag (eg. Windows and Unix+KDE), then the drag should be cancelled; moving the mouse outside the black square should not cause the drag placeholder to continue following the mouse, and the mouse cursor should revert to default.
If the platform's normal behaviour is not to cancel a drag (eg. Mac and Unix+Gnome), then the drag should not be cancelled; moving the mouse outside the black square should cause the drag placeholder to continue following the mouse, and the mouse cursor should show as no-drop or drop-allowed.
Fail in either case if the drag placeholder gets stuck temporarily or permanently over the blue square. Fail if an inappropriate middle click function begins (eg. paste-and-go).