I never found a definitive answer on who blued them... ancient Chinese secret.. With all the stuff swapped, borrowed, crossing countries borders during that time, it's hard to say who did what and when and where it was done.
Only for sure one is Finland.. which is odd, they blued some of their Mosin bolts, but their SVT didn't get blued bolts...... while Russia blued their refurb SVT bolts and even captured Mausers, but didn't blue the Mosin refurb bolts or the SKS, it got BBQ'ed.