if you play the back off spell card hidden do you still draw one?

No — playing Back Off from hidden doesn't draw you a card, because its draw is conditional on playing it from your hand, and a card played from facedown isn't played from hand.

You still get the Stun effect (it resolves at its cost of [0] once played from hidden, gaining Reaction). One restriction to remember: because it's played from facedown, the unit you Stun must be chosen from among units at the battlefield where you hid Back Off, unless another effect loosens that (Rule 811.1.d.2).