1. In the mangaka's mind, is the group the Black Sheep or the Black Bull?
Evidence: The textual evidence according to Indekkusu is: 黒の暴牛, 牛 = Cattle, 羊 = Sheep. The skull is difficult to interpret objectively without input from the author. As evidence of this, neither cattle nor sheep have two widdle white things poking out the sides of their skull-cheeks nor a star on their skull-foreheads. Artist modify things all the time, so the mangaka could have modified the down/inwardly pointing horns of a certain type of cow in a way that suggests a sheep to some people.
Result: Dunno, unless we ask the mangaka over Twitter or something. The text, though, is pretty strong evidence and the art evidence varies from weak to uncertain at this point.
2. In the English translation, what's the most practical translation?
My opinion: Sheep. At this point, sheep. Unless in the future a live black bull gets sacrificed to Yami in the manga, so that you can tell it is a black bull before it becomes a skeleton & skull.
3. In the English translation, what would have been the ideal translation?
Result A, where ideal=ideal in the professional world: Whatever the mangaka would choose when given the choice of two or more options and a good bilingual dictionary plus some pointers from the translator.
Result B, where ideal=abstractly ideal: Whatever would be the phrase that would have the same *impact* in English as the phrase 黒の暴牛 does in Japanese.
Ooo, this is the tricky part. As I said above, 暴牛 = restive bull. So the translation for 黒の暴牛(Kuro no Abarebushi) would actually be Black Restive Bull, *or even* Guilty Restive Bull. According to my dictionary, kuro can mean "guilt; guilty person" instead of /as well as "black" and "black go stone," go meaning the game, so the last doesn't apply here. Restive or Unmanageable certainly applies to the group and would in Japan be a negative trait. (See http://www.etymonline.com/index.php?term=restive&allowed_in_frame=0for the word history of Restive.)
Black as a color in idioms often has negative connotations in English, too, with one of the notable exceptions being "in the black" as vs. "in the red." But because there are exceptions in English, Black by itself won't carry the connotations of Guilty and Restive; neither will Bull, especially together as Black Bull. (Bull by itself has negative connotations as in bull-headed or as a euphemism for bull...crap, but positive associations relating to power.)