Index: b/Doc/Makefile
===================================================================
--- a/Doc/Makefile
+++ b/Doc/Makefile
@@ -6,7 +6,7 @@
 # You can set these variables from the command line.
 PYTHON       = python3
 SPHINXBUILD  = sphinx-build
-BLURB = $(PYTHON) -m blurb
+BLURB = $(BLURB_ENV) $(PYTHON) -m blurb
 PAPER        =
 SOURCES      =
 DISTVERSION  = $(shell $(PYTHON) tools/extensions/patchlevel.py)