Looking at 1980s X-men comic books, I notice so many characters had blue hair, for example, Wolverine, Beast, Mr.Sinister, and tons of no-name extras. Probably this goes beyond the X-men, but that’s the comic I have in front of me to verify this phenomenon. I suspect it had something to do with the available color pallet. Typically the hair is actually blue and black. Does anyone have a definitive answer why so many characters had blue hair?
It's a long-time comics convention, not just used by Marvel. The hair is actually black but to avoid a solid black patch on the page the highlights are depicted in a contrasting colour, usually blue. There was a 1985 DC mini-series called Superman: The Secret Years in which the highlights were brown, not blue, which to my eyes at least looked odd. Here's the cover of the first issue: