Fix subdomain inheritance for nested blueprints.

Fixes #4834

src/flask/blueprints.py

@@ -453,6 +453,15 @@ class Blueprint(Scaffold):
         for blueprint, bp_options in self._blueprints:
             bp_options = bp_options.copy()
             bp_url_prefix = bp_options.get("url_prefix")
+            bp_subdomain = bp_options.get("subdomain")
+
+            if bp_subdomain is None:
+                bp_subdomain = blueprint.subdomain
+
+            if state.subdomain is not None and bp_subdomain is None:
+                bp_options["subdomain"] = state.subdomain
+            elif bp_subdomain is not None:
+                bp_options["subdomain"] = bp_subdomain
 
             if bp_url_prefix is None:
                 bp_url_prefix = blueprint.url_prefix

tests/test_blueprints.py

@@ -950,6 +950,53 @@ def test_nesting_url_prefixes(
     assert response.status_code == 200
 
 
+def test_nesting_subdomains(app, client) -> None:
+    subdomain = "api"
+    parent = flask.Blueprint("parent", __name__)
+    child = flask.Blueprint("child", __name__)
+
+    @child.route("/child/")
+    def index():
+        return "child"
+
+    parent.register_blueprint(child)
+    app.register_blueprint(parent, subdomain=subdomain)
+
+    client.allow_subdomain_redirects = True
+
+    domain_name = "domain.tld"
+    app.config["SERVER_NAME"] = domain_name
+    response = client.get("/child/", base_url="http://api." + domain_name)
+
+    assert response.status_code == 200
+
+
+def test_child_overrides_parent_subdomain(app, client) -> None:
+    child_subdomain = "api"
+    parent_subdomain = "parent"
+    parent = flask.Blueprint("parent", __name__)
+    child = flask.Blueprint("child", __name__, subdomain=child_subdomain)
+
+    @child.route("/")
+    def index():
+        return "child"
+
+    parent.register_blueprint(child)
+    app.register_blueprint(parent, subdomain=parent_subdomain)
+
+    client.allow_subdomain_redirects = True
+
+    domain_name = "domain.tld"
+    app.config["SERVER_NAME"] = domain_name
+    response = client.get("/", base_url=f"http://{child_subdomain}.{domain_name}")
+
+    assert response.status_code == 200
+
+    response = client.get("/", base_url=f"http://{parent_subdomain}.{domain_name}")
+
+    assert response.status_code == 404
+
+
 def test_unique_blueprint_names(app, client) -> None:
     bp = flask.Blueprint("bp", __name__)
     bp2 = flask.Blueprint("bp", __name__)