Unfortunately, updates to the Doc build process doc have not yet been made but the intended process is to provide blurb via the "make venv" Doc build recipe which now also pip-installs blurb from PyPI.  It is the necessary to use that venv to build the docs:

make venv
make html PYTHON=venv/bin/python3

Or you can just install blurb somewhere into your normal shell PATH.  Again, this will be documented.