Blank nodes that are generated while parsing RDF/XML look a bit complicated. A simple example:

?- rdf_load('http://www.gemeentegeschiedenis.nl/gemeentenaam/rdfxml/Adorp').
?- rdf(S, 'http://www.opengis.net/ont/geosparql#asWKT', _).
S = '_:http://www.gemeentegeschiedenis.nl/gemeentenaam/rdfxml/Adorp#_:Description1',

This seems to be structured as follows:

'_:' + <SOURCE> + '_:' + <DOCUMENT-SCOPED-ID>

It would be nice if an alternative blank node prefix could be passed as an option, e.g., I would like blank nodes to look as follows:

<BLANK-NODE-PREFIX> + <DOCUMENT-SCOPED-ID>

The default value for <BLANK-NODE-PREFIX> could be '_:' + <SOURCE>.

It would also be nice if the <DOCUMENT-SCOPED-ID> would simply be an integer. I'm not sure what Description adds here.