Commit d231f36 to fix #3130 added the following check to fields.Field.get_value():

        elif ret == '' and self.default:
            return empty

Maybe I am wrong, but it looks like with this change it is no longer possible to reset a CharField with allow_blank=True but no default argument which currently has a non-empty value back to an empty string: The default value for the default argument is the empty marker. Since the validate_empty_values() method raises a SkipField execption for this marker the field is skipped on update if set to an empty string.

To work around this problem the field may be defined with the empty string as default argument, but to have both allow_blank=True and default='' in the field's argument list seems counter-intuitive.