Utilising interview data with thirty-seven British people of Mixed White and Black Caribbean heritage, this paper draws upon the concept of ‘horizontal hostility’ to describe how Black mixed-race experiences of Black rejection impact on self-perceptions and expressed ethnic identities. In demonstrating the effects of being excluded from a relatable collective Black identity, the paper argues that horizontal hostility is critical in the project of theorising mixed-race. Experiences of horizontal hostility represent significant turning points in mixed-race lives as they can prompt reconsiderations of mixed-race positionings within the broader Black imagined space. Beyond the benefits that horizontal hostility offers to mixed-race studies, it provides insights into conceptualisations of Blackness – as a collective racial identity, community, and politics. The article unpacks how, when and why its boundaries are policed, adding to debates relating to the future formation and maintenance of ethnic group identities and categories more generally.