Materials used in cooktop heating element, and reason for black appearance
The heating element ( on the inside where you cannot see it ) is Nichrome or similar alloy ( 10 to 20 % Cr, bal. Ni ) . The surface sheath is typically Inconel 601 ( 15 Cr, 10 Fe, 0.2 Al, bal. Ni). They were Inconel 600 ( and maybe some still are 600 ) until the addition of a small amount of Al was found to increase oxidation resistance. There are several similar alloys and there are different names for the same alloy. So the black surface is oxides of Ni. Cr with some Fe. Some compositions and temperature exposure will produce a green tint in the black ( in high temperature furnace tubes of the same family of alloys). Diffraction analyses of the oxides ( from furnace tubes) often do not give specific compositions as Cr, Ni and Fe can substitute for each other.