X thinks it is up, but screen is black?

[Tom Horsley]

In my quest for more pixels, I'm trying to get my
new UHD monitor working in 3840x2160 resolution.

The nouveau driver doesn't know how to drive
the GTX 750Ti card, so I had to install the
nvidia binary drivers from rpmfusion.

At this point, everything seems to work except
for the one minor detail that the screen is
always black :-).

X thinks it is up and running, the Xorg log
file shows it recognizing the video card
and selecting 3840x2160 resolution. Nothing
that seems like an error appears anywhere
I've looked. I just have that black screen
problem (Ctrl-Alt-F2 does get me to a console,
so it works fine in console mode - it also
worked in 1920x1080 mode on a different
video source that couldn't support UHD
resolution).

Does any of this sound familiar to anyone?

(I'm off to try stuff like a different cable
in the hope that it is something dumb).