Red Eye

There's a couple of possibilities - none involving the use of flash:

1. The image is edited in an image editor to make the irises look black
2. Wearing black or dark coloured contact lenses.
3. If the iris is already very dark brown, I guess it could look even blacker depending on the lighting conditions (if the light in the room is low, the pupils will be large and dilated). Drug use can also cause pupil dilation.