You could use vickers03's
FOTRSU 1.8 Sub Skins in 4k, grabbing just the Porpoise Black out of there, and then change the "Date \Roster \American \Submarine \SSPorpoise.cfg" entry to only choose the black skin. You also have to change the "Data \UPCData \UPCUnitsData \UnitParts2Porpoise.upc" file to only call the one texture for all the UnitParts sections in the file. You could also keep the Stock file by editing that same Roster & UnitParts cfg & upc files. By 1944 though, most subs were painted light grey on the sides, or were in the process of getting it. That was Measure 32/33 paint. You are wanting the original war scheme of Measure 9... It would be best to create your own mini-mod to experiment with, for easier de-activation if needed...