I don't add anything to the water except to raise some slurry at each step with either a diamond plate or, in the case of the Belgian, with a small slurry stone.

I didn't know that Japanese natural stones are acidic, but that would explain a lot, because I don't keep the blade wet or hone long enough for a stain to develop just by water contact. Do you think that a CF is also acidic? I actually switch around between two Japanese stones, my CF and an Escher or two, so I haven't been focused on which stones make a stain and which do not, but I will do so now.

Thanks for the insights, bros!