Will a door or a trapdoor diminish or block out a light source on the other side of it? If so, by how much, and does the direction in which the door is placed change this effect?
No, doors and trapdoors act just like empty space.
(Source: Direct experiments in Minecraft 1.4.6. I built a tunnel with a torch at one end, then placed various blocks between me and the torch while watching for changes in the
If you want a door which stops light, you must use pistons pushing opaque blocks to function as a door.