Trading songs on the blockchain sounds like a reasonable idea until you realize any MP3 player that would enforce ownership needs DRM now, which it must either

Trading songs on the blockchain sounds like a reasonable idea until you realize any MP3 player that would enforce ownership needs DRM now, which it must either enforce by downloading the blockchain periodically, or periodically talking to some untrustworthy music-industry server that purports to interpret the blockchain but could shut down instead.