I don't have much detailed information about blue brittleness other than ASM Metals Handbook. I read in an old metallurgy text that quick cooling from 400-600F might avoid the problem, but not so sure that it isn't talking about Cr-Ni steels only and their "temper brittleness" instead.
ASM about blue brittleness- "Killed steels are virtually free of this kind of brittleness."
So the dip in a torsional toughness test graph at 400F and the steel doesn't re-gain its strength until 600F is not because of being in the "blue brittle stage"? Because the steels tested are usually the higher quality tool steels, surely killed and many vacuum remelted even.
Being self taught on the subject I've got holes in my understanding of high carbon and tool steel metallurgy. I'll appreciate and help filling them in. ;)
Alvin in AZ (hobby knifemaker) ps- gave up my seat in the fall metallurgy class to someone going for a degree and it isn't offered again until next fall :/