There's been a lot of confusion about how to export blender models for minetest in the past, and many people have struggled to find the right export plugin to make .b3d files that work in minetest. For this reason I've modified a version of B3Dexport.py and we've stashed it in the minetest github.com organization:
https://github.com/minetest/B3Dexport
This version is different than others, in that
(1) it's an older version than the other that you can find on github.com, most importantly because the newer version does not produce coordinates that minetest/irrlicht can properly use.
(2) it contains a modified exporter that (by default) omits inclusion of linked texture files.
(3) it's default export options are set to work correctly with minetest out-of-the-box.
We aim to maintain this exporter code such that it produces working .b3d files with the latest blender and minetest versions. If there are bugs, please submit them to the github issue tracker.