FCPX Caption Editor shows black text on black background
This seems to have been caused by a problem in the incoming iTT file. I was able to solve it by creating a new SRT caption role, selecting all the captions, moving them to the SRT role,  and then moving them back. Alternately, it looks like you can change the caption color  to something other than white, and then change it back to white. More at https://discussions.apple.com/thread/250624181?answerId=251201017022&amp;page=1.