Hi Lakelad, I used to catch blackfish all the time as a boy and young bloke. The most important tips I could give you are:
1. Fish light - no more than 6 lb trace
2. Balance your float (the smallest possible for the conditions) with lead on your line beneath float so that it is only just floating with only the very top of it visible. This way when the fish takes the bait the float will go down without the fish feeling any resistance.
3. Present your bait simply and naturally, not a big bunch but a small strip of weed or cabbage with a half hitch to keep the hook covered.
4. Vary your float depth if your not getting downs. I vividly recall one occasion arriving at a crowded rock spot beneath Waverley cemetery with about 10 people trying unsuccessfully to get blackfish. I had been there the previous night and knew they were feeding at a depth of only 5 feet or so. Bang I was on first cast and the next 4 casts, before I had to tell them to shorten their rig and then they were all onto the fish!
That's not all there is to it, but I reckon this will help!