Weird dark blob on north and west sides of custom model

Hey everyone,

after loads of debugging work I finally managed to make my multiblock with custom models work, but I noticed this weird graphical issue on two sides of my model. There is what looks like a lighting glitch, manifesting as a dark blob, spread out across the entire side of the multiblock, even between the separate block models. Here's some images: 

Not sure what could be causing this. The models are identical, just rotated 90/180 degrees. Any ideas?

Edit: I'm, obviously talking about the bottom models. The top ones are just placeholders.

Edited July 4, 20178 yr by BenignBanana