unfortunately not, it was a dream of mine 30 years ago before they were linked, I would have had to transport the boat by truck between the two, now they are joined it is a lot easier, the Danube or Donau is an equivalent to the Rhine and has carried a lot of traffic up to the start of the Yugoslav conflict and prior to that it was through the Comunist states. It has been done by yacht, it has commercial
Harbours along its length. Like the Rhine it flows pretty quickly. I would do the other German rivers and canals before going to the Black sea. But for me it will have to be in the next life.
ps. There is quite a lot of info on Google