Symbolism, Imagery, Allegory
A trick bag is slang for a situation that initially seems innocent or based on good intentions, but turns out to be trap, involving, say, the blackmailing of a politician or extortion. But in this case, it has a slightly different meaning: The trick is to save White's life, rather than ruining him—it's a classic trick bag in reverse. Black wants to put White in the trick bag by somehow getting him to see that life is worth living.
However, White sees the trick bag as a negative situation. Black's trying to cheat him out of a well-earned death in favor of getting him to stay and endure the shared suffering of humanity:
WHITE: Well, the trick bag seems to have shaped itself into some sort of communal misery wherein one finds salvation by consorting among the loathsome. (128)
By the end, White manages to put Black in the trick bag. He gives Black false hope by sticking around for so long, and then finally demonstrates that his views haven't changed at all: He's still in love with death. The trick bag ends up returning to its classic form, then, with something good turning out ever so poorly.