Blackwater tank on your 30DS ?? would be an bespoke after market installation. If you have a holding tank somewhere on the boat there must be deck fitting to allow the tank to to emptied at a pump out station , however on one of my boats there was a diverter valve and dedicated pump which allowed you to empty the tank directly into the river or sea.
Pretty whiffy it was too.
Locate holding tank and follow any pipes leading out.
On my 30DS there is a macerator pump to a separate outlet above water line. Best way if been in there a while is to add some water (via regular flush) in case a bit dry, helps to shift it. Then you can run the macerator pump once far enough offshore. There is also a black water tank breather (or should be) again in hull above water line.( Mine are both port side as is the heater exhaust) For full picture, normal op mode is as a standard sea toilet with inlet and outlet below the waterline. Both with sea cocks. Diverter valve should be inline on the “out” side diverting waste to tank. On mine the diverter valve is same type as sea cock fitted after a Y piece . so you close the “out” sea cock and open the diverter to use the black water tank, obviously opposite for normal mode.
30ds I have has a huge holding tank, when it's full you can walk across to France....
Yeah probably no tank fitted, mine doesn't and I believe it's 1982, most boats didn't then.
The sucky out deck connection on my boat (which was Broads based) , was non standard. The boat came with real Heath Robinson adaptor bodge to convert my deck fiittng into something like the standard connector on ordinary pumpout stations.