My betta, Cacciato has a black spot on his right pectoral fin. As far as I can tell it isn't bothering him but I'm not an expert. Here are some pictures: http://kichwahya.deviantart.com/#/d3ajpry
He has had this since I first got him from petco about 4 weeks ago. If you want to know his conditions after that and what happened to him you can go to this thread http://www.bettafish.com/showthread.php?t=63454
Just as an update to the current set up he is in a 1 gallon bowl inside the main tank and it is being changed daily.