Well it is a bit complicated to go through, but basically the black sand area is anaerobic. In this area, our "good" bacteria that use nitrate are found. But there are also bacteria that use sulfate...the end products of which is hydrogen sulfide.
Normally this is not really considered to be a big issue, IMO - though it is hyped sometimes. However, if you dug around in the sand bed or moved rocks or something, pockets of H2S could be released, causing trouble as it is toxic.
At this point it is a workable theory in your case.
I would take the whole thing down and start over, personally.