Steel Hardness

[ViggoG]

Thor,
You are quite correct in your observation that the harder steels are slower to take blueing.
This is caused by the finer crystaline structure of the higher carbon steels and the higher concentration of finer carbon crystals that are trapped in the crystal matrix of the steel, which inhibits the penetration of oxide producing chemicals. And the carbon being high on the replacement scale, steals some of the blueing chemicals for itself.
It can be compared to pouring water on cinder blocks and bricks, the finer bricks allow much less water penetration.
(Please, Excuse the poor comparison)
ViggoG