Black (actual name not given) is a really good dude. He lives in a tenement in a poor African-American area of New York, helping drug addicts and other people who are on the skids, and believes strongly in loving your neighbor as yourself. Although he's had a rough life—he's been in prison for murder, been a near-alcoholic at one point, and suffered the deaths of his children—he's still capable of optimism. As a Christian, he believes in a "life everlasting" that can begin now by loving humanity, and in a "most excellent world" existing after death.
Originally from rural Louisiana, Black (who is African-American, by the way, in case you haven't actually read this play yet) has been married, though we don't know how many times. He also has a job, but we're never told what exactly it is. Although he apparently lacks a formal education, he's an intellectual match for his sparring partner, White (for most of the play, at any rate). He has the ability to multiply large numbers instantaneously, too, something he says he learned in the penitentiary and which makes his mental aptitude crystal clear.
Black's sense of humor is pretty distinctive, too. Although White is a little emotionally subdued, given that he wants to die, Black jokes around a lot, kidding White about whether Black people are allowed in his apartment, and clutching his chest acting like White's smarty-pants rebuttals are giving him cardiac arrest.
Those are most of the important external details about Black. But the real man is more inwardly directed—in fact, he's a bit of an armchair philosopher. Whereas White uses his mind to argue for the futility of everything, Black's engaged in the hard work of trying to find some sense in the vast muddle of existence. His conversion to Christianity began at the end of his criminal career: After being attacked and knifed in prison, Black defends himself from his assailant by repeatedly smashing him in the head with the leg from a table that has a screw sticking out of it. Ouch.
Black's attacker suffers permanent brain damage, and Black almost dies due to blood loss. His religious moment-of-truth happens during his recovery when he surrenders to God and hears a voice speak to him, telling him "but for the Grace of God you would not be here." Since then, Black says:
"I don't make a move without Jesus. When I get up in the morning I just try to get ahold of his belt. Oh, ever once in a while I'll catch myself slippin into manual override. But I catch myself. I catch myself." (107)
Yet, Black's religious views aren't entirely orthodox. He admits he's a slight "heretic," but only as much as someone should be, in his view. Everyday he talks to Jesus, who, he explains, "is in my head," after White asks him if Jesus might just be the work of Black's brain. Black is a bit like a Quaker in this way—he believes God addresses him directly, from within (like the Quaker concept of Inner Light) (13).
There's even a vague hint at the beginning that Black might be somehow involved with the supernatural when White insists he didn't see anyone around him when he went to commit suicide, having checked the area very thoroughly. Black makes light of the idea, asking White if he thinks Black's an angel:
"You lookin at some big black angel sent down here to grab your honky ass out of the air at the last possible second and save you from destruction?" (23)
As we get to know Black better, it seems pretty clear that he's a real, fully formed human character as opposed to a supernatural being—though he believes that the supernatural compelled him to be in the right place at the right time, in order to rescue White.
Since his spiritual views help direct the main arc of the play, we should probably get into them in detail: Among other so-called heresies, Black doesn't believe in original sin. The idea of original sin is that, ever since Adam and Eve fell from paradise, humanity's basic tendency is toward evil. Black, on the other hand, thinks people are basically good, and they go wrong by wanting things they're not supposed to have.
The only other heresy he agrees to tell White is his belief that all human beings are, in a sense, Jesus—the presence of Christ forms the essence of every person, that "forever thing." Also (and this isn't supposed to be a heresy), he believes you can have everlasting life—defined not only as life-after-death, but a new way of being that can start now, in this life, if you let your brother off the hook, and stop judging everyone. You need to make mercy your main mode of being in order to experience this, though, which isn't what White wants to do.
In his debate with White, Black wants to convince White that a sense of fellowship with other people is ultimately what'll pull him out of the hole. But White's stubborn; he's never felt any sense of comradeship with anyone else, even other suicidal and depressed people he's met in group therapy. At the same time, whereas White believes in "the primacy of the intellect"—meaning that reason is the most important thing to follow—Black believes in the primacy of God (for starters) and, therefore, of humanity.
But Black's tragedy is that he fails. White simply won't accept his arguments, and Black ends the play in a state of despair, forced to let White venture back out to probable death. Despite this, however, he still believes in God and won't give up on his spiritual mission. He is faithful in the face of anything.