Except for females and their offspring, black rhinos are solitary. Females reproduce only every two and a half to five years. Their single calf does not live on its own until it is about three years old. Black rhinos feed at night and during the gloaming hours of dawn and dusk. read more
This indicates they are at least primarily nocturnal: Eastern Black Rhinoceros. If they’re categorized as such with no explanation, let alone a source, you can add one. Further, an observation that they’re not really nocturnal needs to be backed up by third-party sources; please see Wikipedia:No original research. read more
The word "white" in the name "white rhinoceros" is often said to be a misinterpretation of the Afrikaans word wyd (Dutch wijd) meaning wide, referring to its square upper lip, as opposed to the pointed or hooked lip of the black rhinoceros. read more