mitogen: Clarify blacklisted ModuleNotFoundError message

Previous phrasing was misleading - it implied a given module was explicitly on
the blacklist, even if it was due to a restrictive whitelist and the blacklist
was empty.

Arguably the blacklist/whitelist semantics are themselves misleading. A
redesign is tempting.

docs/changelog.rst

@@ -25,6 +25,8 @@ In progress (unreleased)
 * :gh:issue:`1385` :mod:`ansible_mitogen`: Remove a use of
   ``ansible.module_utils.six``
 * :gh:issue:`1354` docs: Document Ansible 13 (ansible-core 2.20) support
+* :gh:issue:`1354` :mod:`mitogen`: Clarify error message when a module
+  request would be refused by allow or deny listing
 
 
 v0.3.35 (2025-12-01)

mitogen/core.py

@@ -541,6 +541,7 @@ def is_blacklisted_import(importer, fullname):
     any packages have been whitelisted and `fullname` is not part of one.
 
     NB:
+      - The default whitelist is `['']` which matches any module name.
       - If a package is on both lists, then it is treated as blacklisted.
       - If any package is whitelisted, then all non-whitelisted packages are
         treated as blacklisted.
@@ -1536,9 +1537,8 @@ class Importer(object):
         return importlib.machinery.ModuleSpec(fullname, loader=self)
 
     blacklisted_msg = (
-        '%r is present in the Mitogen importer blacklist, therefore this '
-        'context will not attempt to request it from the master, as the '
-        'request will always be refused.'
+        'A %r request would be refused by the Mitogen master. The module is '
+        'on the deny list (blacklist) or not on the allow list (whitelist).'
     )
     pkg_resources_msg = (
         'pkg_resources is prohibited from importing __main__, as it causes '