[Sloeber]

The black is from what is called the anoxic zone, or without oxygen. Essentially, this is present in all reef aquariums, even those without sandbeds. The inner core of LR also has zones of anoxia. These areas of no O2 is where our anaerobic bacteria live and continue the path of breaking down the dissolved organic compounds. By the time the DOC's are finished in the anoxic zone, they are released in the form of a gas (nitrogen IIRC) (the gas bubbles you see along the glass pane in your DSB or suddenly rise from your DSB).

Literally speaking, that black zone is what you are trying to recreate with your DSB. So that's good that you have it. However, I'm concerned with how near the surface you are finding it. This tells me two things - 1) you lack enough DSB "critters" (everything from worms and pods to cucumbers and conchs) to sufficiently turn over your sand and 2) you didn't vary grain size of the sand (guessing you used Southdown exclusively) and the sand is becoming compacted near the surface.

Is this bad? No, not if you don't touch it and let it do it's thing. However, if you go in and stir it all up, that's not good. Everytime you see black, you are essentially burning the anerobic bacteria with O2 (killing it). So don't do that (LOL).

HTH