This is coming from someone who IS legally blind, in his left eye at least, it means that the vision in the eye is so bad, you may as well be blind in that eye, for all it matters. (I might not be entirely right with this definition.)
being blind means you can't see anything (Pitch black) Legally blind means you can see shapes, blurs, and colors, but you still can't see anything. It basically means you can't drive.
