I have purchased a second hand maruti 800(1996 model) car and is in good running condition, but black smoke is coming out from the engine. I took to mechanic and he said that, the black smoke coming out of the engine is due to failure of one cylinder and need to be go for changing cylinder and overhauling the cylinder head. Is it black smoke is due to failure of one of engine cylinder?
If your 3-cylinder, ~800cc engine was only running on 2 cylinders, I would not think that anyone would describe it as being in "good running condition".
Black smoke from a gasoline engine is generally unburned fuel. This can be due to a variety of causes.
If the smoke is coming from the exhaust pipe, the problem could possibly be resolved by a simple tune-up (maybe a clogged air filter, bad spark plug, problem with the fuel delivery system, etc).
If the smoke is coming from the engine itself, then a tune-up is not sufficient--you likely have a leaking or blown gasket, either exhaust manifold gasket or head gasket.
However, the source of the problem may be deeper still, and it may be possible that cylinder head work is required.