Picture wont show unless I leave and return to game.

Hello

I built a new pc recently and am having an issue whenever I try to start a match. When the game begins after champ select the screen goes black and after a few seconds I heard a ping sound but nothing popped up so I hit the windows button and then clicked back into the game and there was the message regarding my firewall. I have been playing league for around two years now and have always gotten that message I simply click retry and it always works so that is what I did and the screen went black again and stayed black. I then herd the new summoners rift sound of the players warping on to the field but the screen was still black so I hit the windows button again than returned to the game and it was back and looked fantastic! I proceed to play the entire patch with no issues whatsoever. If anyone has had a similar issue please let me know.

Thank you