Is there any canon description of this city’s appearance? I am especially interested in the possibility that the Haven might had 'impenetrable black' walls too (since it was a Númenórean settlement older than Minas Tirith, and quite important in its days, and since Eldacar was unable to take it from the Sons of Castamir, only after they left for Umbar).
We don't know
The city of Pelargir is never described extensively beyond the description of it being Ancient and at one time the greatest of the port cities of Gondor. This is reconfirmed by the lack of a map in the Atlas of Middle-earth.
Never are enough details given that allow us to speculate on it's structure, defenses or it's design.