Accompanied by an acrid smell, may indicate that lubricating oil is getting into the combustion chamber. Oil is meant to lubricate the moving parts inside the engine and, by design, is resistant to high temperatures. Although it is a hydrocarbon, like other fossil fuels, it does not burn well inside an engine and much of the oil comes out of the tailpipe in partially burned or unburnt form. The oil also carries contaminants from inside the engine which undergo changes in the combustion process and contribute to the creation of smoking and unpleasant smells. Engines may run satisfactorily while burning oil, except that the oil level must be topped up frequently as oil is consumed. Really smoky vehicles may use as much as 1 litre of oil every 200 km. Normal engines, however, burn little or no oil in 5000 km of driving.
This is a definite indication that this engine is burning oil. The most common cause for this is bad rings and is most likely from high mileage. An engine with bad rings will also show signs of power loss, especially when going up hills. Another cause of blue smoke is bad valve stem seals. Usually the smoke is more noticeable during acceleration. Lastly if your vehicle has a turbo charger then it may possibly have leaking turbo charger seals.