Why was blue hydraulic system chosen for RAT?
Because in A320 is the only hydraulic system independent from the engines and isolated from the other two hydraulic systems (while G and Y have the PTU in common). Also the B hydraulic system is able to control both elevators and ailerons, while Y cannot control ailerons and G controls only the left elevator. That's a design choice taking in account all the possible failures to reduce the risk of a triple hydraulic failure.

There are two switches that allow you to open the RAT:

The RAT MAN ON (on HYD panel), which open the RAT and feeds the B HYD
The RAT MAN ON (on EMER ELEC panel), which has the same effect of the previous one, but it also connects the emergency electrical generator


Please also note that in case of a dual engine loss, the RAT is automatically deployed.