Here are a couple of officially supported, fully up-to-date open source example mods for BL:
- TerrainEdit: a WorldEdit-like mod that allows command line terrain editing. It is written with it's own internal API of sorts, so if you want some practice interfacing between mods then you can get some practice here.
- RealisticStone: a very simple mod that makes stone harder at deeper depths. This one is easy to understand and has a lot of potential to expand, so feel free to modify it how you like.
Currently in development are a few other mods, these will be added when they are complete.