I bought black colorquartz a few months ago and this is how I washed mine. (Wear gloves, btw).
Get a small 5g bucket, put about 5 handfuls in and turn on your hose full blast. You should use some kind of nozzle because you want as much pressure as possible to wash the sand and get all the dirt and debris out.
Keep washing the sand full force and mixing the sand with your hands to get all the black out. Wash for a minute, then rinse it. It'll be very black. Keep washing like this until the rinse water comes out clear. It should take you about 5 rinses to get clear rinse water.
Then just put that sand in the tank and wash more sand until you have enough in your tank. It's a lot of work, but it looks very nice so its worth it.