I ran into this problem while testing X with OpenBSD 4.3: the console went completely blank after firing startx — that is, switching back to other virtual terminals while X was running would only display a black screen. And that was all I got after hitting Ctrl+Alt+Backspace from X, too: nothing but black. The command-line was still there alright, as I could blindly startx again and get X to show up, but the console terminals remained invisible until the next reboot.
The display adapter I was using was an Intel i740 with the driver set accordingly in xorg.conf. I didn’t manage to get the video BIOS upgrading soft (Vflash.exe) from Intel to work, and never found the root cause of this problem. It was probably either the card itself or the i740 driver. I suspect that because once I swapped the card with an extra Radeon 8500LE I had laying around, and the switched to the ati driver, console VTs worked like a charm. Also, with the i740 in place they did work by using the basic vga driver, so my best bet as for the culprit would be the i740 driver.