@k9q1eogcjbx6fy black spots on screens are usually related to a broken screen. We would of course have to see your screen to verify this, but going by your description, it sounds to me like you have to replace the screen [guide|21499|Adding images to an existing question - iFixit Repair Guide|new_window=true]
[guide|160063|iPad Pro 12.9" 5th Gen Screen Replacement - iFixit Repair Guide|new_window=true]