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.