Ring one of the Agents or go into an Independant travel agent. You could start with the freephone number at the top of the page i.e. 0800 091 162.
No need to be pardoned for as far as I can see this is complicated. There is no direct lead in, that I could find, for the Black Sea so the only route for me was, because most of the cruises entering the Black Sea start in the Mediterranean, to look at the Med Cruises and work your way in from there. There better links on other sites so you could use them and work out a price and then come back and check the selected cruise and the price here. Of course someone could prove me wrong and tell you it's easy if you do this or that but I found it difficult .... Nei.
Some cruise companies link Black Sea cruises with those through the Mediterranean. You will have to search thorugh the brochures or websites.
Thomson do Black Sea cruises with the Calypso (adults only small ship) so their website will have information. we are going on this cruise in June and are really looking forward to it.