I understand garbage mattes. The exposure between the extracted matte and the garbage matte can be adjusted by pushing the contrast—which affects only the matte, not the element being matted—but there are trade-offs from this approach. I agree that the STAR WARS shot was not an ideal example.
The image below, from George Pal's THE TIME MACHINE, better illustrates what I meant by mis-matched blacks. There are three elements here: the park on the left, the city street on the right, and the matte painting of the city in the background. Even if the matte lines between the elements were not visible, the "black" levels between the trees in the park and those in the matte painting do not match. (To say nothing of the color difference.) Actually, the blacks don't match between the two live-action elements, either.