by 1993, blue was becoming the industry standard for interaction for hypertext. The operative words here are "industry standard". In other words blue links are

by 1993, blue was becoming the industry standard for interaction for hypertext. The operative words here are "industry standard". In other words blue links are merely a convention, like driving on the left. Many websites don't conform to the 'industry standard', and use colors like red unaccessed links, followed by another 'non-standard' color for an accessed link instead of the purplish color that most websites use.