There is an element of power in the choice of eliminating Albert's last name. Whether we presume that the author (Alice Walker) or the character writing the letters (Celie) makes the choice, eliminating the name is a method of removing Mr. ____ from his position of power by effectively erasing the idea that he has a real human identity.
Mr. ______ finally speak. Clearing his throat. I ain't never really look at that one, he say.
Well, next time you come you can look at her. ...
Mr. ______ say How old she is?
He say, She near twenty. And another thing-She tell lies.
Additionally, this technique is used to lend a semblance of realism to a fictional text in a way that is similar to the use of historical events in a work of fiction. The implication is that the person whose name is replaced by a blank is a real person. Using the blank protects the real person's identity and to a degree separates and protects the speaker from that person.
Walker uses the absence of a last name as earlier traditional literature did. Mr. represents every man for Celie. He is merely one of a monolith of men as she perceives them. He has no separate identity from this group.
Celie's marriage to Mr___ was not based on love - it wasn't even her choice or her will to marry him, she was forced into it. He had no identity for her as a lover, friend, or even husband. He was, for all purposes, her owner, her master. She thought of him as "mister" because that's all he was to her...a dominant male that controlled her. His name was not important to her since his only function was to be her master...therefore, that's the way she thought of him.