I ordered the tonkotsu black for my first visit to sansotei. I didn't think that it was anything amazing though.
I personally thought that other ramen places (santouka, kinton, etc) had more flavourful broths with more depth to the flavour. That being said, I didn't try the regular non-black tonkotsu so the comparison isn't exactly fair.
If you're debating between the black tonkotsu vs. the black miso, definitely go with the black miso. The broth flavour of the black miso is so much better, however I didn't care for the braised pork piece that came with it because mine was all fat. :/