I get that it can be annoying, especially in the level 50 main scenario dungeons like you mentioned, but honestly for most people it stays black for one second, two at the most. So while its annoying, i dont really see it as game breaking enough for them to devote the time to fix it or develop a workaround. This has been here since the very beginning of the game, therefore the coding could be tied to numerous other things.. fixing it could break other things and end up costing tons of development/debugging time that i dont think they'll spend just to prevent people from having a literal second or two of black screen..