A plant labelled as such has not shown any of the brown leaf markings of the "Chocolate" . When does the marking show up?
In older plants ? In fall ? This plant is probably about a year old.

I have chocolate mint, the leaves are just a darker green on mine, smells a little different than other mints.

I have moved the plant from a south winter window to a north winter window. I do see some darkness on the leaves which were away from the light.