Why are Links Blue? ∙ An A List Apart Blog Post
Sir Tim Berners-Lee, inventor of the web, is credited with making hyperlinks blue, a decision he appears to have reached at random. But although accessibility may not have been on Sir Tim’s mind at the time, the color choice was a happy one, according to Joe Clark:
Red and green are the colours most affected by colour-vision deficiency. Almost no one has a blue deficiency. Accordingly, nearly everyone can see blue, or, more accurately, almost everyone can distinguish blue as a colour different from others. It was pure good luck that the default colour of hyperlinks is blue with underlining.
Joe Clark, Building Accessible Websites