In early novels, it was not uncommon for authors to use a blank in place of a character`s name , to create the illusion that the character was someone the reader might know-someone else whose identity had to be kept secret.
What does it mean that celie must call her husband Mr._? When does she at last begin calling him by his firstname?
Mr.____ is really a symbol to Celie. He is a symbol of uncompromising male dominance over women. She is scared of him and what he represents. While under his tyranny, Mr.____ remains nameless because of his unceasing brutality. I don't think it is a secret rather than he is not developed as a human being. It takes Celie leaving him before Mr.____begins to change and turn into Albert. Celie began calling him by his first name after she realized he was not her source—after she was comfortable enough within herself to reject him. When she finally realized they were equals, Shug was the first person to hear the word, "Albert" come from Celie's lips.
http://www.anovelreview.com/2010/07/color-purple-read-along-alberts-last.html
