In the current crop tool, I can select a rectangle using the standard
rubberbanding method.  When I have selected the rectangle to which I want
to crop, I click in it or hit the Crop button and the image is cut down.  

This works OK, but I think drawing the rectangle's outline is not the best
way to really give a visual impression of what the effect of the cropping
will be.  The human eye still sees the parts of the picture that are
outside the rectangle.

This is particularly true if you're trying to crop a photo to get a
particular aesthetic effect or visual balance.  I find that I get better
results by cropping repeatedly and undoing, or shrinking down the window. 
(If you are just trying to cut out a well-defined object then the current
system is OK.)

What I would like is for the GIMP to optionally show the would-be-cropped
areas as blocked out.  The effect out to be like using cardboard
right-angles to try things out before cropping a piece of paper.

As a start imagine that everything outside the selected rectangle was just
drawn in black.  This would help in the situation above, but it might make
it a bit hard to adjust the rectangle, because you wouldn't have enough
context.  

Therefore it might be nice if there was a "preview opacity" selection in
the Crop tool control box.  When this is set to 0%, the behaviour is as it
is at present: the to-be-excluded area is shown unobscured.  When it's at
100%, the area outside the crop rectangle is drawn in the background color
(black, white, checks, whatever.)  In between, the background is blended in
so that you have some context but it is also easier to see how the picture
will look when cropped.

Thankyou for your consideration