def get_bbl(arxiv_id):
    """
    Get the .bbl files (if any) of a given preprint.

    .. note::

        Bulk download of sources from arXiv is not permitted by their API. \
                You should have a look at http://arxiv.org/help/bulk_data_s3.

    :param arxiv_id: The arXiv id (e.g. ``1401.2910`` or ``1401.2910v1``) in \
            a canonical form.
    :returns: A list of the full text of the ``.bbl`` files (if any) \
            or ``None``.
    """
    tar_file = get_sources(arxiv_id)
    bbl_files = [i for i in tar_file.getmembers() if i.name.endswith(".bbl")]
    bbl_files = [tar_file.extractfile(member).read().decode(tarfile.ENCODING)
                 for member in bbl_files]
    return bbl_files