We recently discussed image skeletal structure. The first step for a sound structure is to put something dark in all four corners - even if anticipating a crop. This creates a frame to hold the image together under any circumstance.
Most newspapers place a black border around images to mark each image's boundaries and separate it from other page elements. However, it's not wise to assume the borders always appear on the image.
If something goes wrong during pagination or paste-up, the image's dark corners hold the image together on the page and give the viewer some visual guideposts as to where the image stops and the page begins.
Many smaller black and white newspapers still make image halftones and do not use rule lines. This means all the white areas of a photograph "bleed" onto the page. At these publications, it's critical to put something dark in each corner. Otherwise, parts of the image simply float in a sea of words.
With fine art images, darker corners are almost required. Fine art pieces are frequently matted on white or museum board (cream). The mount almost dares fine artists to try Zone X near the edge.
At least PJs' mat boards are black. In college, I used gray board with a black core (very cool). Now, I have a super-special brass matt cutter with all the bells and whistles. I rarely use it, but someday...
How to make dark corners
Often, a look at the scene dictates what to avoid. Obviously, try to keep anything white or brightly lit areas away from corners and choose to put darker objects into the corners of the image.
Sometimes this may require PJs to crawl under a bush or frame the shot with some other foreground object. Another way to darken the corners is to pump-up the strobe a stop or two and drop down the ambient light (set flash to +1 and meter at -1 or greater).
Avoid afterimage work
Although it's possible and accepted practice to burn down corners or slide the curves in Photoshop, try to avoid it.
Instead, concentrate on making a good composition from the start and the image can go from the camera to the page quicker. Sometimes, there simply isn't time to work the image before delivery.
I'll save potassium ferricyanide treatments and W. Eugene Smith discussions for another day.
Enough for now,