Nick: Why are Renshi belts black on the back but Kyoshi belts aren't?
Andrew: Good question...
Just to make an educated hypothesis... the designation of 'Renshi' is awarded to those who have reached 5th or 6th Dan. Seeing that both of these are halfway to 10th Degree, the visible side is white and red and the 'hidden' side is black - this is to signify the transition of being an 'expert' to 'master'. Once someone has received their 7th Dan, they receive their 'Kyoshi' designation and the black part is then gone.
Now about the white and red combination... White portion represents purity, naivite, etc. and with a person reaching a new level of mastery, technically they are just beginning a new pathway to becoming a master. This is equivalent to the modern day education system - elementary, secondary, post-secondary/undergraduate, Masters degree, PhD, where PhD is parallel to receiving the Hanshi Red Belt.
Nick: I just posed the question to Bill Hayes. I'm sure he will know.
To Sensei Hayes: I hope you had a great birthday and weekend. I was hoping you could answer another question of mine? I was wondering why the reverse side of a Renshi belt is black, but the Kyoshi and Hanshi belts aren't?
From Sensei Hayes: Nick-san: Good question. The first style to use a Red and White Renshi Obi was the Goju-kai established by Gogen Yamaguchi, Sensei, in Japan. The Renshi obi he created was solid Red and White on both sides and was seen first in the States a little about sixty years ago when Peter Urban, Sensei, first brought that form of Goju-ryu Karate to America. The Goju-kai still uses that belt to this day. In that system, 4th Dans wear the white stripe on top and 5th Dans wear the red (more senior color) stripe on top. BTW, they do not use a panel belt in their system.
As I recall, the Goju-kai filed a trademark on that belt design and afterward other systems started using the Red & White on one side and Black on the other side to get around the trademark. Apparently it is also less expensive to make a belt that way.
The whole belt color thing can really get out of hand - as can titles. Some systems award Red and Gold belts and there are still a few Masters who wear Gold belts - quite striking to see.
Hope that helps,
Andrew: #2 is new to me... all Goju-Kai people I know beyond Rokudan don't wear master belts. Maybe it's something no longer practiced.