Opacity is not the same thing as mystery. Mystery refers to information that's genuinely unavailable to you. Opacity refers to information that is (or was) available to you, but is difficult to figure out, notice, or remember.
In Texas Hold'em poker, the fact that you can't see anyone else's hole-cards does not count as opacity, because that information has never been made available to you. If you were allowed to see everyone's hole-cards at the beginning of a hand, and then later in the hand you had to struggle to remember what they were, that would count as opacity.