Coming back to a couple of posts that hinge around the 'pigeonholing' mentality we have as modellers, that sometimes blurs our ideas of what ran with what, either one way or the other
I think I'm right in saying that the North West, Leeds/Bradford and the Bournemouth line are the only significant areas where steam regularly worked blue/grey stock to any extent (please note that I'm specifically excluding things like the Waverley specials). The North East also had steam into '67, but very much on freight workings.
Go back to 1966, and you have many more pockets of steam working (I have an article from either Steam World or Steam Days that reviews that year, and it's fascinating) - but you lose on the incidence of blue/grey stock because it was only in that year that it started to become generally applied
This one works the other way; introduced in 1964 and taking hold in '65, and with many wagons having the lettering on fresh patches as well as full repaints, there are lots of incidences of it in steam hauled freights.