This has happened to me twice during Solo queue. Everything is fine, then after champ select its just black. I then get a "can not connect to the server" message. Attempts to reconnect are never successful. All of the following were tried while the game was still going on.
-I tried restarting PC
-Putting LoL on Firewall "allow" list
-Disabling Firewall
-Removed LoL-Replay
-Watched video where i deleted some files in RAD->solution->releases.
-Repaired LoL encase of corrupted files
-Logged onto 2 separate computers and trying to connect from there. (Both PC's allowed other accounts to connect to games after trying my own account)
I also had two of my friends experience this black screen while in a Ranked 5's game Yesterday. Unable to connect after Champ select.
I can connect to a game easily after the "black screened" game ends.
I don't have anything to help you or me I'm afraid, but you aren't alone. I too would like some help solving this problem