diff --git a/src/flask/sansio/blueprints.py b/src/flask/sansio/blueprints.py
index 38c92f45..231839c7 100644
--- a/src/flask/sansio/blueprints.py
+++ b/src/flask/sansio/blueprints.py
@@ -355,7 +355,7 @@ class Blueprint(Scaffold):
                 bp_subdomain = blueprint.subdomain
 
             if state.subdomain is not None and bp_subdomain is not None:
-                bp_options["subdomain"] = bp_subdomain + "." + state.subdomain
+                bp_options["subdomain"] = f"{bp_subdomain}.{state.subdomain}"
             elif bp_subdomain is not None:
                 bp_options["subdomain"] = bp_subdomain
             elif state.subdomain is not None:
diff --git a/tests/test_blueprints.py b/tests/test_blueprints.py
index 69bc71ad..9f87b290 100644
--- a/tests/test_blueprints.py
+++ b/tests/test_blueprints.py
@@ -966,7 +966,7 @@ def test_nesting_subdomains(app, client) -> None:
 
     domain_name = "domain.tld"
     app.config["SERVER_NAME"] = domain_name
-    response = client.get("/child/", base_url="http://api." + domain_name)
+    response = client.get("/child/", base_url=f"http://api.{domain_name}")
 
     assert response.status_code == 200