I don't think Airstream ever made a blue one, though they did produce a few in color for Wally Byam to stand out as the lead trailer on caravans.
However, Airstream's Argosy division always made painted trailers (originally painted a cream white) and the one you saw might be an Argosy that's always been painted that was painted a custom color by a current or previous owner.
Some people have painted Airstream branded trailers as well, of course.
Look at the upper curved area at the front and back (above the window.) If it's made of 3 or more pieces of metal it's an Airstream. If it's just one piece, it's an Argosy.
Post a photo or two and we'll tell you more than you ever wanted to know.