Hi hahatushka357,
riff_raff is right, and is referring specifically to the Airbus A320 family. Like the A320, the A300, A310, A330 and A340 families of aircraft also employ three independent hydraulic systems, which for convenience are referred to as Green, Yellow and Blue.
On the larger a/c, the Blue system is powered (like the others) by an ED (engine-driven) pump. But on the A320 family it is normally powered by an AC (electric) pump, with a RAT (ram-air turbine) being available to power it in emergency.
Although the Blue system is regarded as the emergency system, it nevertheless contributes to normal operations, including primary flight-controls and the leading-edge slats.
(My knowledge does not extend to the A380 and A350.)