def METHOD_NAME(self, usage_key):
    """
    Adds a bookmark for the block.
    Arguments:
        usage_key: UsageKey of the block.
    Returns:
        Bool indicating whether the bookmark was added.
    """
    try:
        bookmark = api.create_bookmark(user=self._user, usage_key=usage_key)
    except ItemNotFoundError:
        log.error('Block with usage_id: %s not found.', usage_key)
        return False
    bookmarks_cache = self._bookmarks_cache(usage_key.course_key)
    if bookmarks_cache is not None:
        bookmarks_cache.append(bookmark)
    return True