To find the blue key in PC TNT Evilution map 22, Habitat:

From the map's starting position where you first appear,
walk a few paces forward until you come to a stream of blue
water.

Turn left and follow the stream west and south a few paces,
then turn right and head west across the green courtyard.

As you approach the wide wooden door in the west wall, it
should open.

Step into the chamber beyond the door.  The back/west wall
of this chamber is a false, intangible wall.  Simply step
through it, and you'll find the blue keycard on the ground.

PC TNT players can click here to access a demo that shows
how to find the blue key on this level.