When there is no identifying pronoun or relationship should the sex field be marked "blank" for pallbearers and for those conducting the funeral? I haven't been able to find instructions regarding this.
No you would make the pallbearers as "Nonrelative" unless their name is mentioned somewhere else as being a relative.
Correct, the pallbearers field for sex would be marked blank, as would any family member whose gender is not identified. We can't determine gender from names. So, for example, in the sentence - They had 4 children, Jane, John, Marion, and Phyllis - the sex field is blank and the relationship is child.