My understanding is that it had to be changed during filming, as blue stripes on the X-Wings would not have worked with the blue-screen compositing technology that was used by ILM for the special-effects shots. But, for whatever reason, the references in the novelization weren’t changed.
However, in Rogue One, Blue Squadron does appear in the Battle of Scarif – as, by that time, the visual effects were done via CGI, rather than compositing.