Dang. I'm not on Win 8.1 anymore but on Win10, I have gone back to Dan's mirror from the SteamVR mirror since it doesn't have the edge tremors but Dan's mirror works great for captures for me. I haven't tried changing the format of Dan's mirror to a single eye view, though. I'm just doing the standard unwarped stereo view.
OBS is pretty versatile, though. I'm guessing you are aiming OBS at the mirror window and what is in the mirror window then appears in the OBS monitor window but it records black? Or is it black there and it records black - which is what I would expect if the monitor window is black. If the OBS monitor window is black, I'll bet there might be a setting that's not right in OBS.
I'm guessing you are also aware that the free Win10 upgrade is about to run out, too? That's one thing about Win10 is it supports multiple desktops. I move whatever mirror window (SteamVR or FlyInside) over to desktop 2 and maximize it. Gets me great resolution in the 1920x1080 capture.
Wish I was more help. Good luck!