The tool already consumes its own URL blacklist: https://www.wikidata.org/wiki/Wikidata:Primary_sources_tool/URL_blacklist.
Instead of keeping blacklisting, I think a more sustainable direction for this tool would be a whitelist.
This was also suggested here: https://www.wikidata.org/wiki/Wikidata:Requests_for_comment/Semi-automatic_Addition_of_References_to_Wikidata_Statements#A_whitelist_for_sources
Said that, blacklisted references come from Freebase datasets.
StrepHit already implements a whitelist.