Blithesome comes from blithe, a word that has been a part of English since before the 12th century. Blithe can mean "casual" and "heedless" as well as "joyful" and "lighthearted," but blithesome obviously makes use of only the "joyful, lighthearted" sense. Blithesome didn't show up in print in English until 1724, and is now relatively uncommon, but you'll find it in the works of such authors as Charles Dickens, Sir Walter Scott, Mark Twain, and Henry Wadsworth Longfellow.
Examples of blithesome in a Sentence
a blithesome girl who never seems to be sad or angry
a blithesome and silly joke among old friends