You didn't mention how many rustings you have done. I don't know where you are at, but here on the east coast (Pa.) I think it is not humid enough yet to get a good blue. Even in a rusting box, I like to have about 85% humidity.
Sometimes it takes 8 or better rustngs with 6 etchings to get the right contrast. Plus immediately after etching I card under running water with a piece of denim or burlap.

Best English Stub Twist circa 1907


WW Greener c. 1892

David