Small sparks (known as arcing) when plugging in or removing a power supply are quite normal and are related to the inductance of the AC adapter.

Any wire loop (any conductor has small inductance), such as a transformer, has an inductance. The transformer has a magnetic field (or lack of one) and when you plug in or unplug the cord, current flows/stops and the magnetic field changes. Nature abhors a change in flux (change in the amount of mag field) and a counter-current is induced by the inductor. This current causes the spark.

Discoloration of the metal prongs on the charger may result due to this sparking, and are also quite normal.  If, however, you happen to see discoloration on the outlet itself, replacing the outlet is recommended.  And of course, a charger that is physically damaged should be replaced.

As a reminder, laptop AC adapters (aka chargers) should be used on cool, flat surfaces, and not operated in contact with thermal insulators such as blankets.  Just like the laptop, strong airflow and ventilation is required for proper cooling and operation.