Find the blue vent switch was a mission in The Precursor Legacy that Jak and Daxter completed to obtain a power cell. Keira Hagai said that several eco pipelines led towards the Forbidden Temple. Jak then found and activated the blue eco vent switch there.
After completing "Get to the top of the temple", you will have to use the lift at the top to enter the temple itself. Down here there are three passages but two are closed off, forcing you to take the middle one. You will have to jump on top of a few moving platforms to cross bottomless pits and avoid any vines. After three gaps you will find the vent switch. Take the power cell and by doing so automatically activate the switch.
From this point on, any and all locked blue eco vents are now free-flowing. The most notable vent is the previously locked one on Sentinel Beach, required for the mission "launch up to the cannon tower".