If you specifically want black sand I would look into The Tractor Supply Black Diamond Blasting Sand (a lot depends on state taxes but it is less than $10 in Wichita Kansas)
I have always kept my Cory's on a just sand substrate, the color is all down to your choice my friend. There are a lot of different colors out there, if you are not fussy about it being black I would visit the nearest Home Depot or Lowes and pick up a bag of whichever is cheapest. If you specifically want black, visit Tractor Supply.
There are some folks that really like pool sand, some will still use play sand, and yet others swear it makes no difference. Because I had 54 aquariums at one point cost was the deciding factor for me, less than $5 and it would give me enough for 5 or 6 tanks.
How much sand to use? Well that depends on the size of your tank. In a 10 gallon tank I would put a 2" substrate.
In my planted tank (the one above) I would hazard the substrate is approximately 3" - 4".
In my 150 (Clown Loaches, Green Terrors, plus of course a few Cory's) I would hazard a guess at 4" - 5" deep.
One of my 40 gallon breeders has 50lbs of sand for my Corydoras melanotaenia, which is about 2" deep at the front and slopes to about 4" at the rear.
Hope some of this helps