Take a screenshot of the area in question:
Take a screenshot on your Mac - Apple Support
Use preview (or other) to view the screenshot.
If the dark spot shows in the screenshot, the issue is software related. Try to power-cycle/restart the Mac to see if the spot goes away.
If the dark spot does NOT show in the screenshot, the issue is hardware related.
In this case, contact Apple support to see what your options are as far as repair goes. If under warranty, this hardware defect may NOT be covered since LCD panels are allowed some dead pixels as "normal".
If NOT under warranty, this will be an expensive repair since the entire LCD would be replaced.
Official Apple Support