Having looked at the unipac sand I must say that I'm slightly disappointed with it. It's not black? It seems to range for light silver to dull grey depending on the lighting. The only sand I've seen which fits the bill aka is black is "caribsea Tahitian mood sand" which retails for £30 for a
9kg bag
It looks gorgeous but it would cost £60 not including delivery and that to me is by far the most expensive substrate I've come across. Bar the overpriced fert laced, leech heavy, only good for a year, amano type nonsense they sell to planted enthusiasts. But in my case just to cap soil I'm not sure.
The most hilarious thing about it is that in the states it sells for $12-13 (£6-6.50) so I might just buy it from there.