OK Mag, checked it out. The early versions were in matt black, but this soon changed to gloss back, due to the better 'concealment value' offered by a gloss finish. This is also in line with WW2 RAF trials of black paints for night bombers, which started off as a 'sooty' black, to matt black, to a semi-matt finish. The gloss finish had a better absorption effect than matt, making the aircraft less prone to detection, particularly in searhlight beams.
I wouldn't always trust box art!
Note that although it was a gloss finish, this soon dulled down, and wasn't, in service anyway, a highly polished gloss. Also, the P61 showed flaking of the paint along panel joins. Whether this was true of the example being modelled, can only be ascertained by photographic evidence of course.
Hope this helps mate.