Shewolf13 is right, showering or bathing will remove the singed problem. I'm not sure if it will wear off on its own, but it would take quite awhile. While singed, the Sim is at much higher risk of death. For example, if a singed Sim tries to repair something electronic, he/she is more likely to get electrocuted and die from it. Best to get that shower or bath as soon as possible.