If the crankcase ventilation system (i.e. breather) is the source of the oil, you need to attend to that system item by item to see why it is happening. If the source of the oil was the intake manifold, the situation would be much more serious - probably a major mechanical failure. However there would then be wet oil in the combustion chamber. Black smoke in the exhaust is usually a result of rich mixture, while blue smoke is caused by oil entering the combustion chamber. It sounds as if the oil flooding from your breather has clogged the air cleaner, obstructing the entry of air into the engine and causing a massive vacuum in the carburettor venturi, resulting in ultra-rich mixture. You can check this by starting the engine without the air cleaner. If the black smoke ceases, you have found its cause. Fix the problem in the breather system (whatever it is) then wash and re-oil the air cleaner element if it is the foam type.
Joe and Bruce have told you the most likely causes of the breather system problem.
