Mark,
Just a couple of comments.
I have the feeling that the scraping marks are fairly shallow (and I bet that the spots start shifting around from one bluing to the next).
Second, I suspect that you have the tendency of running the blade off the piece, resulting in "rounding the edges", that is, you are tendentially high in the middle and practically no blue spot near any of the edges.
I urge you to sharp your blades, grinding the nose at ~60mm radius. Even if you haven't the perfect grinder, you can get it done in a makeshift way using, for instance, a Dremel tool with a diamond disk, a disk sander with a sheet of silicon carbide paper, or whatever.
It is rather important to achieve a certain depth of cut, having scraping marks at least a couple of tenths deep.
Now comes something that requires you to believe it works, since the results are fairly negligible at the beginning and become obvious only after a few passes: when the overall flatness of your piece is within more or less a thousandth, you need to start to lay down spaced lines with equally spaced individual scraping marks.
If the surface were more or less even, without clear holes without scraping marks, you would scrape it like this:
But, in your case, you should avoid to scrape into the valleys (i.e. areas without any blue):
The part that is hard to accept at the beginning is that, starting from this phase of scraping, you will leave a lot of blue untouched. You'll take care of it the next pass or the pass after that. You need to follow strictly the individual lines and individual scraping marks rules before anything else.
Within a few passes, you will noticing the spots organizing themselves more or less in a checkerboard pattern.
I don't know if you ever had the experience of developing a black and white print in a darkroom: after exposing the paper, you drop it in the developer tray and you start slowly shaking the tray, to make waves of developer, to bring fresh chemicals in contact with the emulsion.
For several seconds (which feel like an eternity), nothing happens. You keep agitating and the paper stays white. Then, almost suddenly, the image appears with all its details within a few seconds.
To summarize:
1) Shape and sharp your blades.
2) Do never let more than half blade leave the edge of the piece, otherwise, you'll round the edge.
3) Have faith that the checkerboard pattern will pop up magically, if you follow the individual lines of individual scraping marks.
Have fun!
Paolo