Did you know that it is possible to get black sand? It's very costly though.
Black sand isn't actually as expensive as you might think. i just got a 2.5kg bag from maidenhead aquatics in plymouth for around the same price as regular silver sand. Obviously it would need more than one bag, but I believe they also do a 7.5 kg bag for about £10. The brand was unipack and the colour was 'limpopo black' It isn't jet black, but looks quite effective... I did find a much darker black sand also from unipack, but it was very much finer than the other, it was almost like mud in its consitancy. I have seachem flourite also which is very expensive, but that is very shrap and no good for corys.. hence the small bag of sand for a top layer