blueing will not add anything IMO
any spray on finish should be removed were the trunion hits.
Id shoot for a .001" interference fit . IMO .002 "is a bit to tight. (yea I know what the internet says stc) I also know what industrial standards are for hardened an un hardened parts under 1" dia an you wont find .002 on any steel 1"or under bore even on unhardened parts . I shoot for.0075" on my personal builds measured with mics not calipers . If I hit .0015 I stop an polish an install.
I would not finish the barrel first an then populate it as your going to scratch it were the parts press on. also metal to metal is how pressed parts should be fitted . If You park it you might find a reduction in size from reports I have heard.
If you can mic out a .00003"change your a better man than I am thats less than heat will affect the part or the tool measuring .
MY best advice is if your doing a spray finish do it after all the barrel parts are on an pinned . then the pinheads an press marks wont show.
Most factory AKs are white under there parts BTW.
I definitely would not add a spray finish between the trunion an recvier or the barrel an trunion it serves no purpose an will scrape off any way. metal to
metal fits dissipate heat better.