The marked face is darker and stands out like a sore thumb.
I have tried.
Changing the layout in terms of 4 sided faces, 3 sided - the dark area was in the same place but bigger before, this is the smallest I have made it.
The faces are in the same plane. I rotated them until they were vertical and used 'P'
There are no faces overlapping each other.
..
Any help is appreciated as I want to get this model textured before I die.