The impact will be very minimal, I believe.

The reasons could be the following,

1. Sulphate reducing bacteria could produce sulphates

2. Generally the colour is dependent on the pH,  is to be analysed

3. As you have mentioned the black color, the following conditions can also  be deduced from you observation like dull water indicates toxic ammonia levels, if dark greenish color is there then algal culture formations.

4. Check if there are any Stinky black sludge at the bottom of the tank. 

5. However, the impact on the output might be minimal as stated by Mr.Saidan

Hope if these could solve your issue

If I'm assuming correctly, "black wastewater" (blackwater?) refers to the source/origin, not the actual colour. I am no expert in the field "efficiency of anaerobic septic tanks",  but I see no explanation, how trapped gases (CH4, H2S, CO2) could reduce anaerobic biodegradation. On the contrary, the layer could help to maintain a low redox potential by preventing oxygen diffusion from the headspace (depending on how gas tight your tank is) into the reaction zone.

How do you come to this suspicion? Having trouble with organic matter reduction?

@chinnusamy vijayanand: Sir, I meant the source of wastewater not the colour of the water. Black water (human excreta + flushing water) is wastewater collected from toilets/latrines devoid of greywater( wastewater generated through other domestic needs:  washing, bathing etc). I'm sorry if I was not clear in question posted. 

@Sandro Castronovo: I had doubt what happens to anaerobic process over time, if the gases accumulate in wastewater due to entrapment by thick sludge layer in completely closed tank with no gas vent option.  

As I already mentioned, I'm no expert in this particular field, but from my understanding it is not very likely that a biofilm layer can be completely gas tight (diffusion). In addition, if your tank itself is completely gas tight, as you wrote in your last post, you would have high gas concentrations in the liquid phase anyway, see 'Henry's law', wouldn't you agree?

Dear Nitish V. Its healthy phenomenon to have a layer of scum.. In practical exp. when for some observations you sight after opening the inspection manholes on the top of the tank.. you sight frothy..film with bubbles emanating.. one has to be careful.. safe procedures to follow. All this inspections are done to effectively monitor the process.. HRT..in each tanks/ process etc..On daily inpections.. At best whisps of vapours are sniffed daily.. to assess the health of the process.. Variance on top layer would be also due the proportion of sewage : sullage, how much flush water is used per use. Any other cleaner chemicals.. used in communities.. etc..  w wshs .. 

To my knowledge, formation of scum layer leads to blockage of oxygen supply to the waste water. So, this leads to the anaerobic conditions. The rate of reduction of organic matter follows the life cycle of bacteria passing through different stages. And as the time progresses, the rate of reduction slows down. I hope this is a valid explanation to the asked question.