Watch for QTDecoder error messages. If you point the Library to a location that has a QT file that Resolve does not like for some reason, it can crash the QTDecoder app. You normally would get a message if that happens. If the QTDecoder app crashes, for whatever reason, any QT type files will show up as black after that. Until you restart Resolve, which also restarts the QTDecoder app.
Something would not be wrong with the BMC files. But if there is a troublesome file any where in a path that the Library view accesses, that can crash the QTDecoder app.