It's an excellent question, and one I've never seen explained - i.e. what exactly does photographic bleach
do (as opposed to why you do it or what effect it achieves, which is explained in a million places.)
I don't know about Cyanotype in particular, but my understanding is this; I'd actually be really grateful if someone could correct me/explain better:
Developing a print turns the silver halides into metallic silver. The fixer then removes any remaining silver halides (i.e. the undeveloped part) which means after fixing your print should be just stable metallic silver.
As I understand it, photographic 'bleach' turns metallic silver back into silver halide. If you were to bleach and then re-fix, you'd remove everything you bleached back to silver halide leaving you with a nice stable metallic silver bleached print.
(This is why all colour processes involve a bleach step before the fix - you want to remove all the silver leaving only the dyes behind, so you bleach the silver image out entirely and then fix in the normal way to remove the whole shebang.)
When you bleach for toning purposes, you turn the metallic silver back into silver halide, but instead of fixing the halides out you 'redevelop' them using the toner; i.e. the bleaching step 'reactivates' the silver so the toner can work on it.
That's my handwaving understanding; I assume the intent is similar with cyanotypes - but like I say, I'd love someone who knows what they're talking about to explain
.
