Or it is just optic effect. Look at the J-20 model, there is the dark region on the left side.
The JF-17 has a polished fuselage and leading edge (left wing).
It could be that, these models are made of metal with matte surface finish. For some reason such as being rubbed in hand, the matte surface becomes polished enough to be reflective. The dark regions are reflecting surrounding objects giveing the feeling of opening. Matted aluminium or brass are quite easy to loose the finish by daily usage, hand, cloth and paper.
The JF-17 has a polished fuselage and leading edge (left wing).
It could be that, these models are made of metal with matte surface finish. For some reason such as being rubbed in hand, the matte surface becomes polished enough to be reflective. The dark regions are reflecting surrounding objects giveing the feeling of opening. Matted aluminium or brass are quite easy to loose the finish by daily usage, hand, cloth and paper.