Isaiah 51:22 KJV says:
Thus saith thy Lord the LORD, and thy God that pleadeth the cause of his people, Behold, I have taken out of thine hand the cup of trembling, even the dregs of the cup of my fury; thou shalt no more drink it again:
God promises to aid His Chosen People, even though they have sinned. He has been angry, no doubt, but they are still Chosen. The same it is for Sonny. He has sinned (with the drug abuse) but he is still the chosen one of the family when it comes to art.
According to the "Biblical Foundation of 'Sonny's Blues'" by James Tackach:
When Sonny's band takes a break, the narrator beckons a waitress and asks her to send drinks to the band members. The narrator watches as the waitress "put a Scotch and milk on top of the piano for Sonny." Sonny does not immediately notice the drink, "but just before they started playing again," the narrator states, "he [Sonny] sipped from it and looked toward me, and nodded. Then he put it back on top of the piano. For me, then, as they began to play again, it glowed and shook above my brother's head like the very cup of trembling" (140-41).
It seems by this biblical imagery and allusion, a kind of holy grail, that Sonny has won his battle against drugs and the older brother, the narrator, will stop trying to control Sonny. The cup is a symbol of reconciliation. The previous post is spot on about the duality of the cup: it was one of fury, but is now one of peace.
I personally have not read the work, but a thesis statement is your main argument. What you want to prove and how you are going to prove it.
If your professor wants biblical allusions, the metaphor of the cup can work in nicely, because the cup is a very important metaphor both in the Old Testament and New Testament. In the Old Testament, the cup is the cup of God's wrath and the enemies of God will be intoxicated with this cup. Also, in the New Testament, Jesus says if it is possible take this cup from me. This is reference to his impending suffering on the cross. It is a cup of suffering and agony, based on love for his people. It is also a reference to this Old Testament motif. Finally, there is a third cup in the New Testament. It is the cup of communion where the church drinks and eats together to remember the work of Jesus and his love.
Now, if you tie this into Baldwin's essay, ask yourself, how this imagery of the cup fits in. And work this into your thesis.