Just fixed this same issue on my TB Display. Disassembled the entire LCD panel to fix the random blank screen issue (solder LED bar connectors. Always the culprit and fixed about 5 of these now) I thought for sure I had a blown LED and dark patch was from residue. My dark patch had a striped pattern (like rail road ties).
Upon full disassembly, found some kind of tape that probably moved with LED generating heat. Peeled off tape that caused the dark patches and used 99% iso to clean off the glue residues. Good as new.
Unfortunately, most people won’t be able to fix this. Require a full tear down of the LCD panel. Tape is on the backlight diffuser plexiglass so its way inside. Reasonable chance to destroy the display unless ultra careful and/or experienced.
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Here is the teardown guide if you really want to attempt this. And if one gets that far, might as well reflow the LED connectors to prevent future blanking out problems common to Apple’s 27” LCDs of this era (iMac 27, Cinema and TB Displays)
https://drive.google.com/file/d/13GqPCJhnHzYTtnmZH2NjAcYVGeeirabr/view?usp=sharing