Today Crazy Redd's little store was open. I saw a sword...and it was the black katana. I needed a certain amount of money (won't tell or it will spoil my auction in the trading post) but I was 1,000 bells short. I had some fish on me, so I sold them and I was able to buy the black katana. Isn't it like the 5 rarest item in the game?!
Black Katana- 12,000 Bells (Not too pricy)
White Katana- 36,800 Bells (Substantial for non power-players)
Doesn't really matter, they are both way awesome items (of the 9 that make the Japanese set of furniture)