Sorry to chime in so late. In my TKD organization we have a "Cho Dan Bo" ranking between the 1st Gup/kyu and 1st/cho Dan. This belt is black and is the end of the colored belt curriculum. It is listed as the last grading on the students grading card. 1st Dans are issue with a dan certificate. This rank learn the WTF form "Koryo" and 1st Dans learn "kum gang". I know this is different to the standard WTF format but I have seen that it works well and the students have a year to extra to learn to 1st Dan. (It isn't for extra money!!!) We have 9 Gup/kyu ranks and then Cho Dan Bo. I have trained in several school prior to starting with this school in 94 due to a group of great and friendly instructors. No school had the same belt colors, but all had 9 or 10 Gup/kyu ranks. I hope this sheds a little light on the plain black belt.