Reassigning to pfeldman@ who introduced the console message in r526624.  I personally think that the current verification (checking response.params.blockedCrossSiteDocument boolean flag in LayoutTests/http/tests/inspector-protocol/network/block_cross_site_document_load.js) is fine and may be sufficient.  OTOH, maybe there is an easy way to test the contents of the message itself (e.g. to verify that the placeholders like %s got correctly populated).