PNB is too busy being resentful for a (whole) bunch of stuff to see the allies before his eyes. It may come with time. On the other hand, he's been around a very long time. What he isn't already today, how likely is he to become that tomorrow? (But to be fair, if Hermione stuffed me in her purse, I'd likely be stroppy, too.)

Portraits are interesting and not very well explored in the canon stories. It leaves a lot of room to play in. (Not to mention plot holes. Why doesn't the wise and so knowledgeable portrait tell you / teach you x? Why doesn't it tell someone else? Why aren't all the pure- and half-bloods using them to 'phone home' from school...)