There are reported cases where gameplay is recorded, but the resulting video is completely blank (or black), but still has the game audio playing. 

This can be caused by several issues listed here:
If you use SLI or Crossfire: Try Disabling it and see if it your videos are no longer black.
Do you use MSI Afterburner?  Disable it and see if it is fixed.
Do you use a Laptop with a Dual GPU?  Set the client in AMD/Geforce Settings to a "High Performance" mode in the Power Settings.
Do you have other games running? It can cause problems if you are Alt+Tabbing between 2 or more games.
Running 3rd Party Software at the same time: Make sure that there are no other programs that are hooking in (Chat clients, FPS counters, other recorders, etc)

If nothing here helps the issue, and the problem is keeping you from capturing footage, please let us know about the issue in as much detail as possible. The best method is to recreate the issue so it's fresh, and then follow the instructions to send us a Support Ticket with Logs attached to them.

How to Send Client Logs