Black and white film is simpler. It's a layer of emulsion in which are embedded tiny crystals of silver bromide. They are a range of sizes, and they constitute the grain of the film.
If a crystal of silver bromide absorbs about four photons in about 1 second, it gets "activated." Sorry my memory is a little vague on this. Anyway, when the developer & fixer chemicals hits the crystals, the non-activated ones wash away (taking the sliver with them).
However, the activated crystals turn to silver metal, and stay embedded in the emulsion, where they appear black. And there you have it as best I can remember.
