Single ions will not have darkness or coloration with which to efficiently emit blackbody radiation.
blackbody in Science
blackbody
(blāk'bŏd'ē) A theoretically perfect absorber and emitter of every frequency of electromagnetic radiation. The radiation emitted by a blackbody is a function only of its temperature.
blackbody in Culture
blackbody definition
An object that can absorb and send off radiation with complete efficiency — that is, it reflects (seereflection) none of the radiation that falls on it. The higher the object's temperature, the higher the frequency of the radiation it gives off.