It seems the Flask repository is not in the current working directory. Since this is a GitHub issue from pallets/flask, I need to work with the Flask source code. Let me provide a patch based on my knowledge of Flask's Blueprint implementation. The Blueprint class is defined in `src/flask/blueprints.py` and the validation should be added in the `__init__` method.

Based on the issue description:
1. Dots are now significant in blueprint names because blueprints can be nested
2. An error was already added for endpoint names in Flask 1.0
3. We need to add similar validation for blueprint names

Here's the patch:

--- a/src/flask/blueprints.py
+++ b/src/flask/blueprints.py
@@ -190,6 +190,11 @@ class Blueprint(Scaffold):
         """
         super().__init__(import_name, static_folder, static_url_path, template_folder, root_path)
 
+        if "." in name:
+            raise ValueError(
+                f"'{name}' is not a valid blueprint name. Blueprint names cannot contain dots."
+            )
+
         self.name = name
         self.url_prefix = url_prefix
         self.subdomain = subdomain