When gold precipitant is black, like after stripping with a sulfuric cell, it's because it's finely divided. I am not sure about Mercury, but I personally wouldn't mess around with it, far too toxic. That's not to say the acids we use are not dangerous, but there are easier and better methods than Mercury.
If I remember next time I precipitate Au, I'll take a few pictures, I have a 100x digital microscope I can snap pictures/video with.
At the heart of science is an essential balance between two seemingly contradictory attitudes--an openness to new ideas, no matter how bizarre or counterintuitive they may be, and the most ruthless skeptical scrutiny of all ideas, old and new. This is how deep truths are winnowed from deep nonsense. -- Carl Sagan
"When I get a little money I buy books; and if any is left I buy food and clothes". Erasmus (1467-1536))