Diatoms are on the top of the sand bed. Sand turning black is on the bottom is a sign of a build up of sulphur dioxide. Large areas can be toxic to the tank if disturbed. I'm not sure this would happen in a week's time, so maybe something else is going here. If the tank is still cycling and there is no livestock in there I would disturb the sand bed where it is black. Use a long wooden spoon or a net handle, etc. to mix up the sand in that area. If you get a rotten egg smell then it was a sulfur dioxide build up.
How deep is your sand bed now?
What was your substrate before you added the LS