This article draws on more than 15 years of research to analyze "Black Mexicans," phenotypically "Mexican-looking" youth who identified as Black during adolescence, used this identity to become upwardly mobile, and then abandoned it in early adulthood. Black Mexicans are potentially iconic cases among emerging varieties of U.S. ethnic and racial life, given Mexicans’ status as a key, usually negative, case in assimilation theory. Most such theory posits that assimilation into Black, inner-city culture leads to downward mobility. To explain how and why this did not happen for Black Mexicans, I propose a sensitizing framework using the concepts of conjunctural ethnicity, emphasizing analysis of racial and ethnic identity in local, historical, and life course contexts; and operating identity, which analyzes identities in interactions and can accommodate slippage in informants’ understanding or use of ethnic and racial categories. Some Mexicans used a Black culture of mobility to become upwardly mobile in the late-1990s and early-2000s in New York, adopting a socially advantaged operating identity that helped them in ways they felt Mexicanness could not in that historical conjuncture, especially given intra-ethnic competition between teen migrants and second-generation youth. This article uses case-based ethnographic analysis and net-effects analysis to explain why and how Blackness aided upward mobility among Black and non-Black Mexicans, but was left behind in early adulthood.