Does the black spot look a lot like this?
If so it is most likely a sunburned polarizing filter on the front of the display. It could be in the display itself, but most (?) of the time it is a plastic polarizing film bonded to the display. If you want to go that far you can disassemble the whole thing, peel off the film, and apply a new one. Then put it back together trying to make sure it is sealed against water
. You can get the polarizing film sheet (self adhesive) on Amazon and watch a generic YouTube on the replacement.