I just finished an ARAM game on Howling Abyss and about 5 seconds before the end of the game my screen started flashing on and off. The flashing ended once the nexus explosion animation ended. The flashing was like a dimming then back to regular brightness and it was fairly quick, probably about one full cycle(from dim back to normal) per second.
The enemy team surrendered and that's about when the flashing started. I also went the whole game without dying, and I thought maybe it's a new feature that happens when either of the two aforementioned things happen (enemy team surrender or not dying)? I'm not sure though.
Unfortunately I could not record it.