/// An RDF `blank node <https://www.w3.org/TR/rdf11-concepts/#dfn-blank-node>`_.
///
/// :param value: the `blank node ID <https://www.w3.org/TR/rdf11-concepts/#dfn-blank-node-identifier>`_ (if not present, a random blank node ID is automatically generated).
/// :type value: str, optional
/// :type value: str or None, optional
/// :raises ValueError: if the blank node ID is invalid according to NTriples, Turtle, and SPARQL grammars.
///
/// The :py:func:`str` function provides a serialization compatible with NTriples, Turtle, and SPARQL: