You could try plugging in an external display - if the blotch is visible on the external display it's likely to be GPU, if not then it's likely to be display. My gut feeling is this is the screen itself though, probably not a lot that can be done about it other than replace it. Possibly not worth it? P.S. Apple didn't make a 17" 2017 model - the latest 17" was 2011. This looks like a 15".