Portra negatives have the same orange mask that color negative films have. This is because the film is designed to be printed on color paper. If you wish to make your own prints on BW paper then you cannot use a VC paper because the mask will intefere with grade selection. Therefore you must use a single grade paper.
Ilford makes a similar film called XP2 which has a lighter colored mask and so is a bit easier to print on BW papers.