I have a keyboard right at my computer, so I played the song noted on the site Charlie Baum just linked. The sheet music is not the same as the tune Marybird is singing.

There's a ski resort in Ontario called Blue Mountain, and there a range called the Blue Mountains in Washington & Oregon. But there's no Blue Mountain in the Valley and Ridge.

But if you are in the mountains, you will notice that the farther away they are, the bluer they look. No matter how long you walk, you will never reach the blue mountains. They get green (forest) or brown (desert) when you come close. Maybe crossing a blue mountain is as impossible as getting true love from a travelling married man.