def METHOD_NAME(blob_key):
  """Return GCS path for the blob."""
  if _is_gcs_key(blob_key):
    return _get_gcs_blob_path(blob_key)
  # Legacy blobstore key.
  blob_info = get_legacy_blob_info(blob_key)
  if not blob_info:
    return None
  return blob_info.gs_object_name