This appears to be another manifestation of a known bug in the current nVidia drivers, which can cause a black screen. I've experienced this glitch with Bianca running Beryl 0.2.0 Emerald, and also with PCLinuxOS 2007 TR3 running Beryl 0.1.9999 Emerald. In both cases, the nVidia driver is version 1.0-9755.
I had a text editor open in X when I did CTRL-ALT-F1 to go TTY. I then did CTRL-ALT-F7 and got a black screen with an I-beam cursor. So, on a hunch, I blindly tapped a few keys and then CTRL-S. After restarting X, I reopened the text file and saw the letters that I had blindly typed at the black screen. Evidently, the earlier X session context had indeed resumed after TTY, but it wasn't displayed because of the nVidia driver glitch.
This test went the same on Bianca (GNOME) and PCLinuxOS (KDE).
The Beryl Manager applet allows easy switching to the Metacity (or KWin) window manager, so the immediate solution is to right-click on the taskbar ruby icon and select Switch Window Manager to enable Metacity (or KWin) before going to TTY, then re-enable Beryl upon return to X.