Blackvoxel is entirely free software. Source and binary are distributed under the GPL V3+ free software licence. This means among other things that get the four fundamental freedoms of free software.
(See the full GPL V3 licence for the full licencing terms : http://www.gnu.org/copyleft/gpl.html )
All this also mean the game can be modded easily : you get the full source code and instructions to build it.
Note : Blackvoxel is said to use GPL V3+ or "GPL V3 or any later version", this mean we permit you to upgrade as you wish to future GNU public licence versions that may be released by the Free Software Foundation in the future.