Most tvout encoders don't support that high resolutions. The max resolution for PAL is 800x600 and for NTSC 640x480. (some cards support higher resolutions but those are scaled back)
Likely the reason the screen was black was because of this. Something else that might be the problem is that you didn't set the tvout format (PAL, NTSC ..). The drivers can't detect what the standard is for your country and because of that it defaults to NTSC which doesn't work on all TV's. If you use a PAL tv I would really set it to PAL (more lines -> higher resolution and image quality).