Noticed no one has answered your post. Could be that none of us know of any river boats that do the itinerary that you mentioned. You can go from Amsterdam to the Black Sea and back - Budapest to the Black Sea etc, but I do not know of or have seen any from Nurenburg to the Black Sea.
We did a cruise a couple of years ago on a Peter Deilmann boat which was OK, but if we had to do it again, we would choise Viking Lines and stay with the newer buildings.... (you would have to ask them if they would do a Nurenburg-Black Sea only segment - maybe they would if they are not totally booked).
If you are use to normal ocean cruising, this ain't it, but it is fun and relaxing. If you would like some additional information, please email me and I would be happy to elaborate on the good and bad parts of our trip.