"Grit" in the film emulsion will be opaque; therefore print white.
Pinholes are actually holes - no density, and will print black.
Some years ago. there was a wide-spread discussion about pinholes and their cause. At the time, a number of film manufacturesrs participated, and the general consensus was that an acid stop bath, reacting with the alkaline developer was usually the cause. I remember Ilford, in particular, advocating the elimination of all acidic stop baths entirely, and substituting either nothing or a water rinse.
I have followed that advice, and in the last six or seven years, have been "film pinhole" (bad) free, except for *one* that cropped up about two weeks ago. I remember that one, because it is totally unacceptable on that particular figure study. Unfortunately, it seems that very little in photography is governed by absolute, infallible, rock steady rules. Translation: Sh*t Happens.
The theory of the use of a stop bath is interesting: supposedly it is used to stop the developing action immediately; but does the development continuing for an additional five or ten seconds really have a significant effect on the final image?
Even if such precision -- note also that whatever effect it had would be reasonably repeatable -- was desirable, as far as I know, no study has ever been done of the comparison of "development stopping times" between an acid stop bath and a water rinse.