What makes an X-ray machine's white photographic film go black? I read on a   bbc gcse bitesize webpage "In older X-ray machines, white photographic film" [edit- I now know that bbc webpage is wrong, and it's not a white film. I'll leave the rest of the post after the next sentence, as is, but note that from the next paragraph, the rest of the post is based on the misunderstanding from that bbc link that there is white film  (And I guess it's talking of any pre digital when it says 'older' - and anyhow prior to film they used photographic plates - glass rather than plastic/film ).]

I understand that X-ray images are black and white. You have the X-rays themselves, the white film, and an object/person in-between.

The white parts of the image are bones and/or things the X-rays couldn't get through, the white film remains white, and the black part is where the X-rays did pass through, and it blackens the white film.

I've also heard that they have a cassette/casing, and in that is fluorescent material or coating and film.  The X-rays hit the fluorescent material and (I guess by absorption and reflection, or absorption and emission) converts it into regular light, and there is a phosphorescent material/coating there, too, so the light remains for a while, and the light, I guess, blackens the film.

I understand they don't work like that anymore and now you can have other types of X-ray detection mechanisms. But I want to understand at least how the traditional ones are black and white.

I understand that X-ray machines used to use a white photographic film.

What is the white photographic film made from that distinguishes it from a black photographic film? Can I buy white photographic film?

I have seen old-fashioned camera film before.

In black and white photography, was it a black photographic film that went white when exposed to light?
X-ray film is very similar to standard black and white negative film. It's basically a transparent plastic base coated with an emulsion of silver halide particles (a salt of $\ce{Ag+}$ with a halide, usually $\ce{Cl-}$ or $\ce{Br-}$). As shown below, when light hits one of these particles, it reduces a small amount of the halide to elemental silver (1). When this is exposed to a photographic developer (essentially a mild reducing agent), more elemental silver is produced around the initial sites (2).$$\ce{Ag^+ +e- -&gt; Ag}$$

Finally, a fixing agent is added which solubilizes the unexposed silver halide so it can be removed from the emulsion, preventing further light exposure from fogging the film.

Thus, the regions that are exposed to light appear dark when developed as the tiny silver particles ($\ce{Ag}$) block light passing through the film and the unexposed regions are transparent, taking on the colour of the plastic base (often blue or grey). X-ray films are typically viewed on an illuminated white background, making the unexposed areas appear white (or off-white, coloured by the plastic base).

Normal B&amp;W film is actually slightly sensitive to x-rays but, the emulsion of dedicated x-ray film contains other components to facilitate the silver reduction from x-ray exposure. 

Source