Maybe it's not exactly Blackmoor (it's the occupied Ten) and for sure it is not canon, but I turned Port Dacoit into a pirate hangout, where the Afridhi leave quite a lot of freedom to the locals (compared to the rest of the lands they conquered, but they keep their eyes wide open) as long as the good citizens of Port Dacoit keep their naval skills well polished and disturb Blackmoor ships (Afresh are not seafarers but they realise that this may be a factor in the war against Blackmoor).
In alternative, there may be pirates protected by the Egg, under the condition that they hand over to the Egg all magic users and magic objects.
There is also an option on the southern coast of Frisia (Bartertown, see:
viewtopic.php?f=22&t=9129) that can be a suitable base.
Perhaps the Iron Duke may decide to turn Erak into a base of privateers, hoping they will become a thorn in the side of the rebel Northern Marches...
If you definitely want the port to be based within the borders of Blackmoor, then maybe Jackport or Heronport (from the MMRPG lore) may be decent options, with some minor cover up from the authorities.