Who wrote the first blues song?

Robert Johnson was the first Bluesman. After there are Big Bill Broonzy, Willie Dixon, Lonnie Johnson......... Muddy Waters , Buddy Guy, ...Hendrix... .
Have a good day.

W.C. Handy is often credited with the first blues song, "Memphis Blues," in 1912. But two other blues-like compositions were published in sheet music that year, according to wikipedia: "Baby Seals' Blues" by "Baby" F. Seals (arranged by Artie Matthews), "Dallas Blues" by Hart Wand. Report It

In actuality, the blues were more likely originated well back into the 19th century around the New Orleans area. See the excellent essay, "Hear That Long Snake Moan" by Michael Ventura (Whole Earth Review, 1987). Report It

http://en.wikipedia.org/wiki/Blues

There's loads of info here.
I've not read it all myself, but you may get pointers to other sites.
Hope it helps.

Some 'bluesologists' claim (rather dubiously), that the first blues song that ... Mamie Smith recorded the first vocal blues song, 'Crazy Blues' in 1920.

May be...

Adam and Eve.

As they were thrown out
of Garden Eden.....


Aaron.

Maybe the Biblical character "Job". If he had musical and lyrical gifts he certainly was qualified based on periods of his life. (If you are confused read the book of Job from the Bible.)
Actually I believe "blues" music is based on African "work songs" that were brought to America on the slave ships from Africa.