Black does make them "pop". Although IMO, most sand looks nice. Also IMO, yes I personally think sand looks nicer. Now, do realize everything sits on top the sand (food, poo, ect), it doesn't go down and inbetween like on gravel, so you will notice stuff more on the bottom, but a quick vac takes care of it.
I just put pool filter sand in my 20 gallon extra high. The footprint is 20 inches by 10 inches, and with that, I have more than half of my 50 pound bag left. There is an average of an inch and a half over the entire bottom. Take into account, I have a piece of driftwood with a slate base under the sand, so there is a little bit of sand displacement there.
I've read that an inch and a half or less mostly defeats the bacteria pockets. Also, some snails or digging fish will take care of things for you.