Blue smoke came out of the exhaust pipe

1995 Toyota Camry LE. A mechanic put in a catalytic converter then only after it was put in, a few days later blue smoke came out of the exhaust pipe. The smoke only goes out for ten seconds after I first start my car but only once per day, regardless of the temperature outside. I thought this prevents smoke going out especially since smoke didn’t go out before it was put in as far as I know. A year later I had a mechanic put in fuel cleaner, fuel filter, oil filter, gasket and motor oil but the smoke problem still exists.

Is there something other then an expensive repair, as the mechanic suggested, to be done? What are your thoughts about what the mechanic told me? He said, “I recommend valve job and valve seals to be replaced. Engine might need to be rebuilt based on inspection after cylinder head is removed.”

Also what would be a normal cost for parts and labor for a mechanic to do a valve job and replace valve seals? Would this stop the smoke?