Heres an interesting article that quickly touches upon the visually impaired. This article "The Blind really do hear Better" gives finding on research on the age old question: do those who cannot see have there other senses hightened? (eg. hearing) This info can prove useful when thinking of headset devices for our core class.
Also the basics of visual impairment (eg. degrees of blindess, defining legally blind, etc.) and statistics can be found at the braille institute.