Total blindness from birth is actually quite rare. Most people who are 'registered blind' have a severe visual handicap but can generally (at least) perceive 'shadows' (as MattK suggests).
However, anyone who is completely blind from birth, such as David Blunkett or the award-winning broadcaster Peter White, receives no signals to the brain from their optic nerves. So, for them, there are no 'shadows' or any other perceptions of 'light' and 'dark'.
Blind people, like everyone else, have imaginations and (within the restricted field in which their imaginations can operate) will have their own idea of what things 'look' like. Depending upon the ability of others around them to describe things in ways that make sense to a blind person, the 'images' in a blind person's imagination can be anywhere between incredibly accurate or completely wrong.
For many years, I ran a Cub Scout pack at a school for visually handicapped children. (This was in the days before Ken Livingstone decided that it should be illegal to feed the pigeons in Trafalgar Square). Every one of those children knew about birds and could tell you lots about them but none of the completely blind kids had ever 'experienced' a bird. I will never forget the expression of sheer joy (tinged with just a certain amount of fear) on one young lad's face when we got him to stand in Trafalgar Square, holding out a container of corn. First one pigeon landed on his arm, then another one landed on his hand and tred to fight the first one off. While this was going on, another pigeon landed on his head. I am absolutely certain that, through the sound and feel of those birds, that boy acquired some sort of 'mental image' of what a pigeon is but, inevitably, it can't have been the same type of image that a sighted person has.
Chris