yes...the both of you are correct, H2S are formed from decomposition...if the SB detritivours are of appropriate amounts, the amount of H2S produced is reduced...but however, if a pool of H2S does get accumulated, then that area can be a death zone, thats why the need for critters to prevent that from happening
on the other hand, H2S will dissociate in the water to become a weak acid and will react with oxygen to form sulphuric acid, but marine environment is usually an alkaline one...and also a weak alkali...so the reaction rate is slow...if less oxygen does reach these regions, the weak acid cannot become sulphuric acid and cannot react completely so the tendency to accumulate till horrible levels
now what i am trying to find out is...how to conclude that the black regions are hydrogen suphide when hydrogen suphide is colourless??