I was at the C&C Party in the "Jails" in Mannheim, 90 kilometers from Frankfurt, look http://www.jails-mannheim.de/
It is like germanfucker described. It is a quite large fetish cellar bar with a lot of couches, slings, toys and so on. It is once per month, starts in early evening. But in the Jails are much more partys. They are not all definitely BB, but at all partys I visited a huge part of the guys fuckes BB.
Nearer to Frankfurt is the No Limit Party in Mainz. It also is once per month, in a quite big bathhouse. It costs € 19,- but is more comfortable. It begins Saturday 6.00 p.m. and you can expect actually ca. 120, 130 guys from a great region, even guys from France, Belgium and the Netherlands. When I was there nearly the whole night there were still fucking. The half of the guys leaves then when they had enough fun. The other half can take a luxurious brunch at Sunday 7.00 a.m. which goes over hours and is included in the € 19.-. And a very exciting thing at that party for me was a great room used as a dormitory like in a boys' boarding-school where the most guys later were sleeping, but at the same time over hours some guys still went on fucking like in a live porno.