A blue puff of smoke at low idle take off

I have a 2006 Subaru Impreza WRX Sport Wagon with only 30K mi. I take very good care of it - oil change every 3K mi. After the last oil change I have started to notice a puff of blue smoke from the exhaust only after the car is warmed up and idling very low. If I goose it a little to get the idle up it doesn’t do it. Ideas?