I believe there is no way to change to a different tty due to the N900's limited keyboard. Afair the key combination to change ttys can be changed in the kernel config, which might be worth considering.

Anyway, if you have Maemo installed in parallel you can access the logs from there.
If you already have network up when the blackscreen appears, you can ssh into the N900.
If everything else fails, take the µSD card out of the device and check the logs on a different computer.
If you installed DebiaN900 on your internal emmc, install another instance of it on a µSD card (WITHOUT Gnome , Lxde with slim works fine), boot from there and check the logs on the emmc.