The uri blacklist has been introduced with SMW 0.5 to prevent certain uri's (such as RDF and OWL language constructs) do not appear as data within the RDF export.
An email1 from Sep 2016 clarifies that:
... The purpose of these was to ensure that the RDF export of SMW is well-behaved and also valid in OWL. Many tools would find it hard to digest if you would declare a property to be, e.g., a subproperty of rdfs:range.
Unfortunately, rdf and rdfs in particular contain a lot of things that may not all be problematic. However, rdfs:label, for example, should be declared an owl:AnnotationProperty, whereas SMW would declare it as owl:DatatypeProperty ...
- MediaWiki:Smw_uri_blacklist purpose? on what is the use case of MediaWiki:Smw_uri_blacklist?