10. Jason is being payed by the pixel, and is slyly trying to pad his paycheck.
9. She has a very small black hole in each eye socket. No light can escape.
8. The light parts of the nurse's eyes were stolen by Shanna to give her that neat sparkly effect. That's the real reason Shanna is being held prisoner.
7. She accidentally applied too much eye makeup.
6. The nurse has cybernetic implants allowing her head to double as a magic 8-ball. If you ask her a question, the words "It appears unlikely" will appear in each eye-socket.
5. The eyes are the windows of the soul. She has the curtains drawn.
4. Those aren't eyes. Two small, black beetles are living on her face.
3. She had a terrible industrial accident, and replaced her eyes with black marbles.
2. She was very naughty, So Santa Claus gave her two lumps of coal instead of the colored contact lenses she asked for.
And, the number one NON reason that the nurses's eyes appear as black voids:
1. Dave Sim's "All destroying void at the heart of every female" is showing through.