A Blue Door, or a Blue Hatch in Metroid: Zero Mission, is a type of Door from the Metroid and Metroid Prime series. They appear in every Metroid game except Metroid II: Return of Samus and are the most common door, requiring no special weaponry to open; a single blast from Samus' power beam will suffice. They are commonly placed between passages in order to block unwarranted company. In Metroid: Other M, Blue Doors are replaced by doors which open automatically whenever Samus nears them.
In Metroid Prime, Blue Doors appear quite frequently. This game includes Missile Door Locks that require Missiles to be blown off; behind the Blast Shield is a standard Blue Door. Blue Doors along with other Doors may have been erected by the Chozo or possibly Space Pirates in some areas.
Metroid Prime 2: Echoes
In Metroid Prime 2: Echoes, the Blue Doors function the same as the Blue Doors in Metroid Prime.
Metroid Prime 3: Corruption
In Metroid Prime 3, some minor changes have been made to the Blast Shields in the game. In this game, there are both Missile-based Blast Shields and Ice Missile-based Blast Shields that are used to prevent access to the Blue Door underneath. Not much has changed from the Blue Door functionality from the previous installments.