You are trying to register the module and not the contained Blueprint
object.
You’ll need to introspect the module to find Blueprint
instances instead:
if mod_name not in sys.modules:
loaded_mod = __import__(EXTENSIONS_DIR+"."+mod_name+"."+mod_name, fromlist=[mod_name])
for obj in vars(loaded_mod).values():
if isinstance(obj, Blueprint):
app.register_blueprint(obj)