The brick can suck up a lot of heat, the brick can hold a lot of moisture, it would not surprise me if it cracked, or would spit chunks of concrete at you when you do heat it enough.
it is hard to tell from the picture but it looks like the flame may have carbonizing flame (lack of oxygen) which is not really bad for silver and normally somewhat desirable, if the torch can produce enough heat to melt the silver at that setting when running with deficient of oxygen, but with a torch just barely able to produce enough heat to begin with, running the torch rich which will produce less heat it may not produce enough heat to get the job done.
Does the torch leave things looking black a sooty?
Mapp gas and a good torch, you should be able to melt the silver, how dirty is the torch you are using, that can also be a problem if the torch is so dirty it is not operating at full potential.
First I would forget about using the building block as some kind of furnace cover, or having the graphite crucible sitting on concrete, if you have nothing else try sitting it on a few layers of sheet rock cut to small blocks and stacked onto each other, the sheet rock will not act as such a large heat sink.
Check the torches flame clean or adjust it if needed, you should have a good volume of bright blue flame, even a brighter blue cone at the base of the flame, with maybe with a little orange or yellow dancing on the flames tips.
you may need a refractory material to sit the dish on like ceramic wool, or soft fire brick refractory (which tend to reflect heat not absorb it as much as the harder more dense refractory will) you may even need a small enclosure to contain a little more heat instead of trying to heat the world around your dish, (atmosphere, or the heat sink materials in that area)....