When two people fuse through the fusion dance, or the potara earrings, the newly fused product leans to the characteristics and personalities of the two former subjects, but they do not submit to their control. The only character who ever submitted to control was super Gogeta, who had no choice.
The pupil is a hole located in the centre of the iris of the eye that allows light to strike the retina. It appears black because light rays entering the pupil are either absorbed by the tissues inside the eye directly, or absorbed after diffuse reflections within the eye that mostly miss exiting the narrow pupil.
The pupil of the eye is actually just an opening to the eye that leads to the lens; no color is possessed by the pupil itself. We see black when we look into the pupil because light that is accepted into the area is fully absorbed by the eye tissues on its way to the retina.