SRD said:
Mind Blank:
The subject is protected from all devices and spells that detect, influence, or read emotions or thoughts. This spell protects against all mind-affecting spells and effects as well as information gathering by divination spells or effects. Mind blank even foils limited wish, miracle, and wish spells when they are used in such a way as to affect the subject s mind or to gain information about it. In the case of scrying that scans an area the creature is in, such as arcane eye, the spell works but the creature simply isn t detected. Scrying attempts that are targeted specifically at the subject do not work at all.
I'm a mean-spirited bastard, and I'd rule that Mind Blank "protects" you from the effects of a Commune -- including the "getting a question answered" effect. If your deity can't find your mind, She isn't gonna be able to leave a message. (Literally, you are "protected" against "information gathering" -- just as a fence might protect you against velociraptors.) So, there will never be an "Unclear" answer regarding the veracity of a Commune.
As to the Defendant paying XP, I'd ignore the core rules. The Goddess of Justice will grant Her priests a special version of Commune that draws XP from a voluntary subject... or even a Rod of Answers which anyone can use, casts Commune on command X/day, and will magically transcribe both the question and the Deity's answer on a specially prepared scroll of parchment. Thus, the question and answer are out in the open, and the Defendant pays the XP. (I'd imagine that frivolous lawsuits are not uncommon, and cause duels -- who wants to pay a portion of his soul just to defend against a fraudulant charge?)
I'd imagine that in a LE land, it would be permitted to use a Hollow Rod of Answers, which allowed you to fill it with Soul Larvae or Liquid Pain in place of draining your own XP, or hiring someone else to hold the Rod while you ask the questions.
Finally:
SRD said:
Commune:
You contact your deity or agents thereof and ask questions that can be answered by a simple yes or no. (A cleric of no particular deity contacts a philosophically allied deity.) You are allowed one such question per caster level. The answers given are correct within the limits of the entity's knowledge. Unclear is a legitimate answer, because powerful beings of the Outer Planes are not necessarily omniscient. In cases where a one-word answer would be misleading or contrary to the deity's interests, a short phrase (five words or less) may be given as an answer instead. The spell, at best, provides information to aid character decisions. The entities contacted structure their answers to further their own purposes.
Note that D&D Deities aren't omniscient. I'd say that, even ignoring my "positive cheaper than negative proof" law, the particular deity may have ignored those who weren't doing anything interesting -- thus, She may not be able to "clear" those who are innocent, so long as they weren't in Her view at the time.
For example, IMC the Goddess of Death, Knowlege, Magic and Law would probably be the one to contact in a murder trial. Let's say person A was just sitting at home playing his guitar all day. Music isn't in Her perview, so She ignores him utterly. Person B was crafting a new spell. Magic is one of Her interests, so She watches her. Person C is Mind Blanked, and actually commits the murder. If you look at the names alphabetically (and meta-game like an S.O.B.), you're going to come to the wrong conclusion:
"Did person A do it?"
Unclear.
"Did person B do it?"
No.
"Ah-ha!"
-- N