The original May 1977 Siskel and Ebert Star Wars review

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.

You can see the same problem on R2 during the Death Star assault - the whole time he’s in the x-wing, his blue panels are black, because the star field background is overlaying the blue on his head.

It’s because the novel came out six months before the movie. By the time they realized the problem on set, the manuscript for the novelization was already at the printers.