pupil of the eye
睛 centers on the pupil of the eye, the dark central opening that lets light in.
睛 combines 目 (eye) with 青, which likely contributes the sound and the idea of clear or pure, suggesting the clear dark pupil.
The eye 目 on the left and the clear blue-green 青 on the right together suggest the clear dark pupil at the center of the eye.
For セイ, imagine saying 'say' while pointing to the pupil of your eye: say -> セイ, and the pupil is the part that 'says' where you are looking.