Any material would work providing you size it correctly for it's specific gravity ratio to gold. Your black sands might not be of uniform mineralogy. The key is to figure what size gold you are ready willing and able to successfully clean up*. Use screens to size accurate. Then use larger screens ( based on density ratio) to create your uniformly sized ( and hopefully uniform specific gravity) test material.
The more precise the better, but using 5.2 to 6(?) for your blacks and 19.3 ( if your gold is pure) gives you about 4x for your test material.
You sorta need to know the density of your test material to size it right. I chose quartz ( silica ) sand in that example because it has a known density and is easy to find or even make. Crush & screen. Of course that varies in nature as well, even in glass. Keep in mind you do not know the purity of your target gold either. Well drilling suppliers sell graded clean silica sands.
This is why we take home concentrates and finish it under more controlled conditions. If you run your sluice clean of everything but gold you have let most it pass right off the end.
*Take your gold you already have. Determine by running through screens what size is retained on the smallest screen. After that nothing matters because you have not yet been able to successfully process that from stream bank to jar. If that is not much % of your total gold, move up your screen sizes until you have an acceptable take at the small size. Might as well be cost effective and achieve an ROI. Calculate your test size ratio based on that. Why waste your time with anything smaller if it will not put much gold in your jar at the end of the journey?
If there is no risk of missing a nugget (pick that size for each place you hunt ) classify the living daylights out of your feed stream into the sluice and run it like a precision machine. You can always pour out the oversize and detect it.
Size matters. Hydraulic separation is about size and density. The more uniform size the better the density separation. Conversely with density and size, but what is the point of that?
Go a bit smaller if you think that in the future you will want to process your old waste concentrates for finer gold. I play with various hydraulic separation inventions and I'd love to have a crack at peoples waste concentrates.