Ok, we reproduced the issue successfully.
It seem Windows 7 take the image of the terminal background rather than the displayed image when you press the "print screen" key. Windows 8 and Linux takes correct screenshot of the rendered surface. Windows XP is untested.
As we plan to suppress the terminal in the future on the windows port of the game (Blackvoxel is developed on Linux) it will hopefully fix the issue.
For now, you can use the following workarounds that we tested are working good on Windows 7:
(1) Use windowed mode for taking screenshots in blackvoxel rather than fullscreen mode. You might have to eventually crop the image to take out the borders. If you need to get back the mouse cursor, use the F1 key (F2 to lock it back).
(2) Use a dedicated program like Fraps. That's what we use. It is not only good at taking screenshots of games and getting fps informations, but it takes also good video without taking too much CPU time. The program isn't free but there is a free version that can be sufficient for occasional use.
The Blackvoxel Team