The blue door in Doom is the first of a series of colored doors that requires you to find the access key of the same color. Here’s where to find it.
When you defeat the horde of Demons waiting for you in the large open area full of rocks, you’ll follow a path along to a door that’s locked. If you get up close enough to it, you’ll notice that it requires the blue access key in order for you to open it. So let’s find that key and keep on with the slaying of demons!
Just to the right of the door you should see a little gap in the rocky surface. Simply head through here and follow the path round. Toward the beginning of this path, you should see another smaller gap in the wall. You can’t get through this but it does give you a perfect view of where you need to be going, a bloodied corpse on the floor. Carry on following the path around and jump down the ledge.
You’ll now take a sharp left turn and will find yourself by the pillar that the aforementioned bloodied corpse was leaning up against. Simply walk up close to the body and hit the R3 button. Your character will swipe the access key off the body and you’ll now be free to open that door.
Go back the way you came and head over to the door and it should automatically flash up with ‘Access Granted,’ before allowing you through.
Now continue on your journey into the depths of hell!