BLT is a Composer package that acts as a development and testing framework for Drupal sites. I do a lot of work on BLT, so I need a way to hack on it locally while also seeing the real-time effects on a Drupal installation.
All of my Drupal / BLT sites also use DrupalVM for the local development environment, which actually makes this a pretty easy task.
Let's say that my local git checkout of BLT is at /my/package/location. All I have to do is define a new NFS mount for DrupalVM like so (in box/config.yml):