The use of blank spaces in place of names as Alice Walker uses them in her novel often serves the purpose of privacy or protection of identity.
This technique is commonly used in non-fiction to maintain the anonymity of a person in a true-to-life story. In this way a writer can be completely honest without concern for embarrassing the individual being written about. This technique can also help avoid the possibility of libel or criminal misrepresentation against a real person.
Using this technique in fiction serves a number of purposes, different from those in non-fiction.
In fiction, the blank space creates a semblance of reality in the text akin to references to episodes in history in a narrative. The blank spaces make the story seem more real; more like non-fiction.
She say Mr. ______ was taking somethin out his billfold to show Pa an it fell out an slid under the table.
Another purpose, which must be surmised here, is that the blank space replacing the name of abusive males helps to place Celie in a more powerful position, erasing the power or dominance of the men that attempt to control her life. On the other hand, the blank space for a name may show her continued fear of that dominance and control: don't speak the evil name and it won't arise again.