The Weary Blues is about a man who is in a jazz or blues bar, playing the piano and singing the blues. That is what the poem is about at its most basic level. Hughes wrote this during the Harlem Renaissance, a time in American history where in Harlem and other cities were African Americans were concentrated, great literature, art and music were created. Blues was especially loved during that time period, and the people would go to the bars and coffee houses, and listen to and sing the blues. It was a way for them to vent their stresses and worries, and to take a load off after a hard day of working to live and survive in a world that was against them.
If you have ever felt down about anything, you can hopefully relate to the poem. Especially if you really get into music, and if music helps you to feel better and vent your emotions. That's what blues were for these people, and Hughes describes this man, his passion, his intensity, and relates how the music moved all of them.
The themes of the poem are being alive through music, the power of music, and the common suffering of the black people during this time period. I hope that those thoughts help a bit; good luck!