require a non-empty name for blueprints

CHANGES.rst

@@ -35,6 +35,8 @@ Unreleased
 -   Ensure subdomains are applied with nested blueprints. :issue:`4834`
 -   ``config.from_file`` can use ``text=False`` to indicate that the parser wants a
     binary file instead. :issue:`4989`
+-   If a blueprint is created with an empty name it raises a ``ValueError``.
+    :issue:`5010`
 
 
 Version 2.2.4

src/flask/blueprints.py

@@ -190,6 +190,9 @@ class Blueprint(Scaffold):
             root_path=root_path,
         )
 
+        if not name:
+            raise ValueError("'name' may not be empty.")
+
         if "." in name:
             raise ValueError("'name' may not contain a dot '.' character.")
 

tests/test_blueprints.py

@@ -256,6 +256,11 @@ def test_dotted_name_not_allowed(app, client):
         flask.Blueprint("app.ui", __name__)
 
 
+def test_empty_name_not_allowed(app, client):
+    with pytest.raises(ValueError):
+        flask.Blueprint("", __name__)
+
+
 def test_dotted_names_from_app(app, client):
     test = flask.Blueprint("test", __name__)