Uh . . . you're in Castle Oblivion throughout the whole game.

Are you on a high floor? Have you tried tripping to lower floors to find the blue card? I found that when I was getting frustrated like this, the best thing to do is to travel all the back to the beginning or at least until you find that card. This may be your only option if the floor you're on isn't getting you anywhere.
. . . besides, most of the doors requirements are randomized, right? So pretty much going back to a previous floor may change the requirements.