Depends on what causes the blindness. My friend went from completely sighted to totally blind. He sees black. He told me the stages he went through. Blurry, outlines of people to black. He told me in his mind we will never grow old he'll remember us how we looked last time he saw us.
Depends on type and level. True blindness they see nothing so i guess blackness. Theres a whole spectrum up to 'legally blind' on the other end where either things are extremely blurry and/or their field of vision is severely limited.