A blown car engine can be the result of several factors and often ends in a broken connecting rod or a broken valve. Although both scenarios result in an unusable engine, the engine does not really blow up.
If, for any reason, a moving part in an engine is unable to move, it may warp or break. When a connecting rod breaks, the crank can push it through the engine block, stopping the engine. Oil can escape through the puncture, resulting in blue smoke, and may even catch fire. If a valve breaks, it can damage a piston, causing oil to leak through the piston and burn in the cylinder. In this case, blue smoke will exit through the exhaust.
A third possibility, and one that is not as serious, is overheating. In this case, a failure to the car's cooling system causes the fluid in the radiator to overheat, resulting in white steam. Drivers can minimize damage by pulling over and turning the engine off immediately.