I've been trying to record some games or printscreen some end results but they keep turning out black. That's sad because some matches are really epic and I'd love to save them

I suppose it's no longer prohibited to share visual content of the game since it's open beta and I've seen multiple channels already, so there must be another cause. I'm playing on fullscreen. Can someone help me?
If there's any additional detail required I'd gladly share it!
Cheers, SubjectSeven