In 17, the black accent was an option available on most colors from the factory, but you mostly see it on black cars. My best guess is that it was the same in 15 and 16 model years. Eventually you will be able to get a Marti report on your car and then you will know how many were made with that color combo.
As soon as the Marti reports are available for the S-550 mustangs I want to get one for mine, I already know that grabber blue was a one year only color, but how many convertibles with a manual trans and the performance pack were made? I'm guessing not many.