Blue Door

Blue Doors (also known as Blue Hatches) are doors that can be opened by any weapon. They are found in every Metroid game except Metroid II: Return of Samus, Metroid Fusion (the latter is replaced by a Blue Hatch), and Metroid: Other M. The blue color is actually the color of a force field covering the door. After shooting a blue colored door, the energy covering it will disrupt and peel off or dissipate, depending on the model. The main purpose of the energy covering the blue doors is to avoid accidental/unintentional entries to rooms by wild creatures surrounding the area. This blocks off a large quantity of bioforms from access since most of them can't manipulate weapons or simply don't have the means to produce an energy-based projectile.

In Metroid Prime 2: Echoes, the logic behind this design of door is explained by using the scan visor on a regular blue door. The extensive explanation is not given when scanning a black, white, or 'grey' door, instead, it only informs you of what type of weapons energy can interact with the field on those doors.

In Metroid Prime 3: Corruption, by scanning a blue door on Elysia (once inhabited by the Chozo), it is revealed that since the discovery of said planet, the Galactic Federation used the model of the Elysian blue door as a basis to all currently existing doors found throughout the galaxy on planets, ships, and stations under Federation control.

Super Metroid manual
"These doors can be opened by Beams or Bombs."

Metroid Zero Mission manual
"These hatches can be opened with any of Samus' weapons."

Trivia

 * In a collection of Scan Visor panels in Metroid Prime, there appears to be an unused scan of a Blue Door that heavily resembles the Super Metroid sprites. Doors are unscannable in Prime, except for the doors on the Frigate Orpheon when Samus first enters it. When Samus scans the doors, the scan will tell Samus which room the door leads to. The reason for this is unknown, and no other doors have similar scans.

Gallery
Blue doors often vary in size shape and appearance, as demonstrated below.