My rebuilt small block chevy has blue smoke on start up,runs fine.?

Old school way to seat rings was toss a little Bon-Ami cleanser down the carb. I dont think that would work today considering the exotic alloys rings are made of.
I dont think the problem is rings anyway since it only smokes at start up.
Probably valve guides and/or stem oil seals. If so it will suck oil through those on deceleration.
You'll need someone driving and someone watching the tail pipe. Back out of the throttle from around 2000 rpm and less it roll for a good 5 secs or more. Step back into the throttle and get a small puff of smoke , there's your sign.