Houses heated by electricity often have a mix of electric baseboard and in wall heaters. When in wall heaters are present, there is often a blackish discoloration or "sooting" on the heater grill and surrounding wall.
Look at the heater grill cover and you will often see this condition present. This discoloration is caused by lint and debris on the grill and heating element. This build up results in excess heat being retained and will eventually cause the "sooting" condition.
Brian Jovag, owner of Jovag Home Inspection.