Spinning Tail would do 80 damage to the Defending Pokemon and 30 damage to your opponent's Benched Pokemon. Darkrai Lv.X's Poke Body says that all basic Darkness Energy attached to your Darkness Pokemon will count as Special Darkness Energy instead. Ir you have 5 energy attached to Tyranitar, then 30+50 is 80, so 80 damage would be dealt. The Benched Pokemon still takes 30 damage because the text says that the damage effect only works on the Defending Pokemon. In other words, the bench is not affected.
Spinning Tail is not an effect of attack. The text says, "Does 30 damage to each of your opponent's Pokemon." Since the word damage is used, this is not an effect of attack.
If the words "damage counters" was used, then it would be an effect of attack, and Darkrai Lv.X would not affect that.