Black is mentioned as the colour of mourning in several places.
Upon hearing of the death of her sister, Procne:
induiturque atras vestes
put on black clothes
Ovid, Metamorphoses, 6.568a
Althaea did the same when she saw her slaughtered sons in the temple - 8.448
Again, when Vitellius heard of the defection of his soldiers:
having thrown on a dark toga
came down from his palace, surrounded by his household in tears, his son carried in a litter as if in a funeral procession - Tacitus, Histories, 3.67
The black toga of mourning and its exchange for a white one after a ritual bath and then worn at a banquet (mentioned in the above answer) can be found in Cicero, Against Vatinius, 13.
An interesting image is that of the last threads of a dying man's life on the Fates' spindle in Martial as:
Lastly, Tibullus writes of Persephone's warning to him of an imminent death:
at mihi Persephone nigram denuntiat horam
but Persephone announces to me the black hour