When is the blue hydraulic system pressurized?

The blue hydraulic system is pressurized automatically with the first engine start because the hydraulic pumps associated with the engines are activated as part of the normal engine start process. When the first engine is started, its associated hydraulic pump begins to operate, thereby pressurizing the blue hydraulic system. This feature ensures that the aircraft's hydraulic systems are functional and ready to support essential systems such as flight controls, landing gear, and brakes, immediately upon engine startup.

The other options may not provide accurate conditions under which the blue hydraulic system is pressurized. For example, while turning on the blue electric pump manually would indeed pressurize the system, it is not the automatic or primary means of pressurization during normal operations; instead, it's a backup method. The PTU (Power Transfer Unit) operates to transfer hydraulic power between systems, but it does not directly pressurize the blue hydraulic system upon being turned on. The priority valve is related to system management and prioritization of hydraulic pressure but does not initiate pressurization in the same manner as engine start.