This is indeed a backdoor. BlackFrame is the name of the frame in the Roblox Loading Screen, however looking through its source, it is not on line 717. The loading gui is often used in backdoors to try and hide the fact that code is running and they try and fake the game still loading. I would suggest looking for “require”, “getfenv”, or “il1”.
Ok no threat, this is a loading issue with Roblox. It has been an issue with them for a while. This is not a backdoor. It is an issue with a CoreScript