I personally believe that H2S will leak out but its not enough to adversely affect our tank inhabitants. In fact, you see all those poisons that corals produce, you will think twice about H2S. You can smell it, yes, but you know its critical when it makes you drowsy. And like what Alentino said, proper set up sandbeds do not have any trace of the "black stuff".
Anyway, analysis results coming soon.