Like you I have travelled by steam train through the hills and watched as the loco took water at Blackwood. At no time on my trips or motorcades did they use a siding leading off the down main at Blackwood; the reason it wasn't there. My memory is backed up by the "Pull Diagram" published as a topic in this forum a few months ago. ( I have just checked and the diagram is no longer accessible; maybe SAR523 could resurrect it.)
The water column Justapax refers to was against the down main at the down end of the platform and locos did not have to leave the train to use it.
A major shunt I did see at Blackwood occurred when 526 had to push a failed 300 set from somewhere near Eden hills to Blackwood. 526 made a good job of it with a trailing load of 6 centenary cars on a school holiday special to Bridgewater. 526 pushed the 300s to the down platform where the 300s pax detrained. the 300s were then pushed out along the down main and the train left in the down platform while 526 then drew the 300s back into the back platform (road 3). 526 then ran around the 300s along the up main before crossing back to the down main to the return to the train and to take a much need drink.
With the train and the 300s the load would have been 300 tons.
I didn't get that out of a book or any other memories as Brian has implied.