Miracle Eye removes a Dark-type Pokémon's immunity to Psychic-type attacks and restores the target's Evasion stat to normal.
So, Miracle eye would do regular damage on pure Dark types because it removes the immunity so it would not make sense to leave a resistance. Though if the Pokemon is a duel type and the other type has a resistance to Psychic (like Bisharp) then it would be not very effective. Also if the the Pokemon is a duel type and the other type has a weakness to Psychic (like Scrafty) then it would be super-effective.
Still Nothing, I think. Mircle eye does not get rid of resistance.
EDIT: I think It is Regularly effective.