ive had many tench out of there, the "blackness" is down to the clarity of the water and the fact that the silt that covers the bottom of the lake is anaerobic. The fish darken up cross their backs naturally as a form of camoflage to help them blend into the substrate. All freshwater species darken up in clear water
There has been a mirror carp around the 28lb mark in there since i was fishing it back in the early 80's .i have recently seen a picture of this fish when it was caught last year so i know its still knocking about
As for the 30lb + commons, unless someone can show me definitive proof then im sceptical. Although the water is rich enough in natural life to sustain a handfull of large fish.