There might be an easy solution for the following problem:
I had been having this issue for almost 24 hours:
My loading screen goes totally black right after champion select.
I never used any 3rd party programs, i reinstalled and re-patched 4 times already, i even delated files forcing a re-patch manually. I have no firewall or antivirus blocking. My operating system is OS X Yosemite. Already sent a ticket to riot 20 hours ago but still no answer.
The cause might be me trying to spectate a game (i have no idea if Sona was in or not).
If you have the same issue, try the following: When you get the black screen, wait a few seconds and PRESS "ENTER" (that big key with a big arrow on the right side of your keyboard). You should get a normal loading screen with all champions after that, and you get in game to play normally.
Try it in a custom game, and if it also works for you as it worked for me, feel free to add me if you wish, and let's try to compensate any lost IP :)
See you on the Rift!