[WIP] My Dearest Pig

[Darkfire]

Definitely use more lighting. When you inspect the original Dark Descent maps, even the ones that seem pitch black (like the prison) have big point lights, but colored brown or almost black.
Second thing is that the player only will notice the darkness if they have some contrasting light, either in previous room or map, or even in the same room.
Seriously, use light. It's pretty much the most developed thing in HPL2 engine.

As for other advice:
Ever heard about decals ? They're somewhere on the left bar in the level editor.
They are things that kind of "stick" to other textures (on walls, floor, ceiling...), they can be blood, dirt, hay, footprints, writings, holes and wood defects etc.
In practice: you need to put a considerable amount of dirt stain decal everywhere to make seemingly empty places (like most of the floor) much more interesting to look at. Make sure to change the "UV SubDivisions" to 2x2 or higher so the texture isn't cut in half.

You also said:
"This is a Custom Story for Amnesia:The Dark Descent. The Machine For Pigs game might be required to play this."
That doesn't work this way. It only works for Amnesia: Justine, as it is installed INTO the Dark Descent (DD for short).
However DD and Machine for Pigs (MFP) aren't interconnected with each other in any way. Therefore you'll need to place all files from MFP that you used in the Custom Story folder, so that DD can read them