Blot Out targets only the opponent, not any permanent. A permanent with hexproof may be exiled this way if its mana value is the greatest, for example.

Consider all creatures and planeswalkers the target opponent controls as one group to see which one has the greatest mana value. For example, if the opponent controls a creature with mana value 3 and a planeswalker with mana value 5, they must exile the planeswalker.

Which creature or planeswalker will be exiled is determined as Blot Out resolves.