There are many happy blues songs. B.B. King's "Sweet Little Angel" for instance, or John Lee Hooker's "Boom Boom". A song doesn't have to slow tempo or be about "My baby done left me" to be blues. It's all about the style. It's the way you sing it and the way the instruments are played. Sort of like the difference between rock and jazz or heavy metal. It's about style. You don't have to be angry or hateful to play heavy metal. It's the way you play the music that makes it metal.

That's my opinion anyway. Of course, it couldn't hurt if your woman just left you for your best friend.