I have to say I'm a bit surprised. That is only the tenth sale of a black NGC that I know of since 1998, and the third this year! (The ten sales only represent 7 coins though. One piece has sold three times. It won't sell again because I own it. I also know of two more coins, but they have not sold in the past nine years. There have also been two sales of broken empty holders. The empty holders tend to bring around $50 - $60.)
I am also a bit surprised at the price. The guy got off cheap. Hadley mentioned the Cincinnati that sold in March for a $3000+ premium for the holder. The other piece that sold earlier was an 1881-O dollar that sold in late Jan early Feb and it also sold for a $3100 premium for the holder. (I know one collector of slabs who had a standing offer of a $1000 premium over the coin value for a black NGC holder before the first sale this year. His comment after the first two sales was that he was going to have to up his standing offer.)
These were only used for about the first month of NGC's operation and estimates are they produced in the neighborhood of 20,000 of these slabs. They were not popular. So when the white slabs came out, a lot of them were reholdered. Then add to that 20 years of crackouts, and resubmissions because of the "Early slabs are good upgrade candidates" mindset and the toll on these has been huge. Many dealers and collectors have never seen or even heard of the black NGC's. Guesses are that there are somewhere between 40 and 400 of these slabs still in existance. (Interestingly the next two NGC varieties are even rarer than the black NGC, but they are not visually arresting like the black one is so very few people know about them and they can be quietly picked up on the rare occasions when they appear.)
I wonder who bought that Norfolk? I know of one collector who is trying to put together a set of all the NGC varieties all with commemorative halves in them. Last I knew he still needed a black NGC.