OK now that I've realized you have to close the tear off in order to save it (durp), I can also confirm that it saves a black image in preview render, but not in SF or FF.

But on the same note, it takes the same number of mouse clicks to save a tear off as it does to export from the arrow 2 icons away from the tear off icon, and I can't find any difference in quality of the saved image otherwise. So... ?

The bug should be reported. But I feel like the priority level would be pretty low on this one considering it only affects preview renders and there are 3 other image export options (file menu, right arrow menu, recent render menu) that don't result in a black image.

Should also be noted that some bugs do only affect certain hardware configurations. That doesn't make them any less important, especially for those experiencing them.