Forming magnetite on cast iron pan
I think the two steps that are missing from you approach are "carding", where, after immersion in the boiling water, the steel is rubbed/brushed to knock off the larger rust structures and repetition. The process is repeated several times before the piece builds up a stable blued surface.

Check out this video of Larry Potterfield applying a rust blue surface to a rifle action and barrel: https://www.youtube.com/watch?v=VPD3GWqk-yE