Compositional　heterogeneity　is　commonly　observed　in　mixed　bromide/iodide　perovskite　photoabsorbers,　typically　with　minimal　effects　on　charge　carrier　recombination　and　photovoltaic　performance.　Consistently,　it　has　so　far　received　very　limited　attention　in　bromide/chloride-mixed　perovskites,　which　hold　particular　significance　for　blue　light-emitting　diodes.　Here,　we　uncover　that　even　a　minor　degree　of　localized　halide　heterogeneity　leads　to　severe　non-radiative　losses　in　mixed　bromide/chloride　blue　perovskite　emitters,　presenting　a　stark　contrast　to　general　observations　in　photovoltaics.　We　not　only　provide　a　visualization　of　the　heterogeneity　landscape　spanning　from　micro-to　sub-microscale　but　also　identify　that　this　issue　mainly　arises　from　the　initially　formed　chloride-rich　clusters　during　perovskite　nucleation.　Our　work　sheds　light　on　a　long-term　neglected　factor　impeding　the　advancement　of　blue　light-emitting　diodes　using　mixed　halide　perovskites　and　provides　a　practical　strategy　to　mitigate　this　issue.　©　2023　The　Authors