A "song" is music. Tune, melody - semantics really. As others have said, lyrics without music isn't a song, it's a lyric or a poem.

I do believe that infringement cases have gotten out of hand, and I absolutely do not subscribe to the notion that chord progessions or rhythmic 'feels' should be covered under copyright law, but the melody is absolutely something that should be copyrighted, and infringements on them should be protected.

As for blues, a great deal of it is in the public domain, and is fair game (for good or for bad).

http://www.rogerbrownsongs.com
http://www.nashvillesongwriters.com/roger-brown