When Annie calls Alvy in the middle of the night and asks him to kill a spider for her, he gets there in no time. There are two spiders in Annie’s bathroom (which he tries to kill) but he finds something else, too: a bar of black soap.
According to Erno Laszlo, it was their Sea Mud black soap.
Later in the film, after the couple breaks up, Alvy is clinging to the memory of the black soap so much that he carries it with him. He even shows it to an old lady he meets in the street.
World Wide Aura 
I’ve just ordered my first Erno Laszlos lol, hope they work for something, I wonder if they are worth it.
I hope they are! What have you ordered?
So far just the Phormula 3-9 balm, but I plan on buying the soaps and the phelityl one later on
I’m lemming the black soap so much. The whole line looks so luxurious (and expensive, sigh).