I am not sure about a truly blind person, but depending on what type of blindness you have will mean you can see different things. For example, my nan has macular degeneration (like on the 3rd Christmas Lecture) and she can only see very blurry images which are all dark and shadowy. Some other forms of blindness give you tunnel vision and others mean you can see light and dark shapes.
As to the second part of your question – is there such a thing as nothing? If you can’t see anything it is because light is not getting to the rods/cones on the back of your eye for the brain to process.