Comets don't reflect light, they generate it moving against solar wind and radiation. After those things are left behind it becomes a just a big ol' flying rock again.
To expand on this a little - the solar wind vapourises part of a comet, forming an atmosphere of sorts called a coma (and the characteristic tail) and this atmosphere glows both by reflecting sunlight and by ionisation. So yes, the cometary nucleus hardly reflects any light, but some light is reflected by the coma and the tail.
(see en.wikipedia.org/wiki/Comet#Coma_and_tail, and just above that for details of light reflection from a cometary nucleus)
