[Veronica]

I cooked some chick peas as per the pack instructions - boiling them in water with a spoonful of bicarbonate of soda. After an hour of boiling the water turned completely black and the pot gave off a smell of rotten eggs. What was this substance? And what would have been contaminating the water to produce this substance by reacting with bicarbonate of soda?

[jcjlf]

Undoubtedly the peas will have contained proteins with the amino-acid cysteine as one of the building-blocks. That would explain the rotten egg odour: H₂S was produced. But still it is strange that H₂S is produced while you increased the pH by adding sodium bicarbonate, which reacts alkaline in water.
HCO₃^- + H₂O  H₂CO₃ + OH^-
The freed (hydrolysed) amino-acids from the proteins could have neutralised this bicarbonate.

And then the black colour: no idea, but wasn't it ordinary carbon? A thermal decomposition of the organic substances?
What material was the pan made of? Iron or steel could give off iron ions that form a black precipitate with sulfide-ions:
Fe²⁺ + S^2-  FeS(s) (is black.)