When these characters were first created, the name was not "Monokuro Boo", it was "Monotone," with "mono" coming from "monochrome" and "ton," a word for pig.
In line with the character's concept, the name "Monokuro Boo" is also a derivation of the words "Monokuro" ("Monochrome") and "Boo" (the Japanese onomatopoeic equivalent for "oink").