I didn't make it!
I have completed all the quests to open Black Lake but I don't know where it is. I don't see it on the world map and I have wondered around most of the docks and the merchant district and can't seem to locate it.
You will have to speak to the City Watch guards at the gate to Blacklake in the Merchant Quarter. The gate is behind the City Watch office (Merchant Quarter) and just slightly right of the entrance to the Park. You should be able to locate the gate on your mini-map.