Worn piston rings or valve seals can allow too much oil to leak into the combustion chamber, leading to oil fouling of the spark plugs. Any fouling is likely to cause at least a little bluish exhaust smoke. Replacing the valve guide seals might help and it's not too big of a job. If the rings are shot, however, that's major overhaul work.
Also have a compression test done on the cylinders. If the compression rings are shot, the oil rings are worn out too. If the exhaust valves don't seal properly, leading to low compression, that could lead to a false conclusion about the rings so you need to look at the exhaust color, but leaky valves can cause misfire issues too. On such an old vehicle, even though it doesn't have huge mileage, bad valve stem seals are a common issue as the rubber hardens with age and heat, so that's what I'd focus on first.
To do the trouble shooting on the valve seals notice if you are getting blue smoke at idle when the engine is cold. At idle the manifold pressure is high and what happens is the oil gets sucked past a bad valve seal because of the high vacuum. If you can't tell at idle, the best way to diagnose it is to create high manifold pressure by driving down a steep grade with your foot off the accelerator pedal and have someone follow you to observe the exhaust color. .
Sometimes the valve guides can be bad too and this will cause blue smoke continually during engine operation. Quite often, a valve guide problem will involve only one cylinder and you will know because only one plug gets fouled. If this is the case, remove the valve cover on that side and then pull the rocker arms and shafts on the offending cylinder(s) with a good pair of plastic jaw pliers to see if you can get perceptible sideways movement on the valve stems. If they seem tight, go with with the stem seals as being the problem.