RAT on A320 deploys automatically if both AC buses are lost - if both engines fail, this is certanly gonna happen.
Blue hyd pump is powered by AC1 bus only, it can never be powered by battery.
If you lose both donkeys and you're left with blue hyd (pressurized by RAT) only, the RAT stall at 100kt will be pretty much irrelevant to you, as flaps will be lost and slats will be available up to config 3 or 22° (they can be set to full, or 27°, but it's not recommended with no flaps) and even empty A320 would stall above 100kt in that config.