Can cement backer board be used to support bluestone or slate outdoor raised patio?
I would say the poured step, with proper expansion gap, however, PT lumber with the proper sub-floor/underlayment then backer board. 

If you just use backer board, then tile/stone/etc, you will have the "floor" flex every time you walk on it. Crumble, crack, crumble, snap and you fall through. Water and backer board are not a good mix either. It is not concrete, but cement so it has no structural component. Backer board is the rough surface that allows the grout to adhere. 

In other words, if you build a box step, use the same technic you would for an interior floor. Then seal it with a sealer to protect from water as best you can.