If you want visible ambient, but you want a spooky-type atmosphere as well, making the ambient somewhat purple might help. (I'm really no fan of grey ambient)
Edit: some more thoughts...
The layout seems fine, I myself only know the basics tho.
I'm no fan of the long corridors, they can be shrunk safely in some parts of this map (by moving whole sections of the map a bit.
I think you really need more detailing, but thats usually a step for when it is sure that other problems are solved. Especially the ceilings get real problems with tiling when the ambient is higher.Also more lighting options rather than just torches might help, such as more openings in the ceiling.
The dark "look" is mostly fog actually, i would maybe have the same fogcolour as ambient. While experimenting, blu-ish and low purple ambients worked the best, even though they might look somewhat odd.
Sometimes, parcour seems too... ineffective to traverse this map, probably because the rooms are this gigantic, not sure if this needs to be fixed though. (Some pushers might help? Maybe a teleporter? not sure)
The map needs classic-weaponspawns.
(Random:I like how the map is broken up in very big cubes, that should make changing layouts and such quite easy, good mapping practice)
Overall, good first effort
There might be more stuff, but my brain is empty for now.