Play your guitar, boy,
Runs out tomorrow's
And evil old
Ain't no more!
The blues, since their invention, have been a form of healing and expression for the downtrodden, particularly with roots in the experience of oppressed African-Americans. This tight poem, which is not lyrically representative of a blues, is an expression of hope and encouragement, even if it is unclear if Hughes actually thinks "hard luck" will ever be no more.
"Hard luck" on a personal scale can be anything: the loss of a job, the infidelity of a lover, financial trouble, drugs. I think in a larger cultural context, "hard luck" can refer to the raw deal African-Americans have received since they were first brought to the US. Post-reconstruction era life and into the highly segregated world of Jim Crow laws was brutal "Hard luck" seems mild, but really, the blues are an outlet for the soul.
Imagining misfortune as a black cat makes for a handy visual representation. He came in yesterday (tragedies of the past, a history of oppression), but with hope, he will run out of tomorrow's back door. In the future, when he, "evil old hard luck" is vanquished, then you can stop singing the blues.
Really though, hard luck will never be dead for anyone, and I think that must be a good thing. Without struggle, what is life? How much art would we be deprived of today if no one ever struggled? Art is the human way to cope with struggle, suffering, beauty, loss, all those things which make us who we are.
I do wonder about the use of the word "boy" which often carries a charged racial connotation, but rather than being demeaning here, I see it as affirmative, an older black man encouraging the younger generation to keep singing the song of struggle, to keep on keeping on. I mostly think this because it's Langston Hughes's poem, and not anyone else's.