Bright　and　efficient　blue　emission　is　key　to　further　development　of　metal　halide　perovskite　light-emitting　diodes.　Although　modifying　bromide/chloride　composition　is　straightforward　to　achieve　blue　emission,　practical　implementation　of　this　strategy　has　been　challenging　due　to　poor　colour　stability　and　severe　photoluminescence　quenching.　Both　detrimental　effects　become　increasingly　prominent　in　perovskites　with　the　high　chloride　content　needed　to　produce　blue　emission.　Here,　we　solve　these　critical　challenges　in　mixed　halide　perovskites　and　demonstrate　spectrally　stable　blue　perovskite　light-emitting　diodes　over　a　wide　range　of　emission　wavelengths　from　490　to　451　nanometres.　The　emission　colour　is　directly　tuned　by　modifying　the　halide　composition.　Particularly,　our　blue　and　deep-blue　light-emitting　diodes　based　on　three-dimensional　perovskites　show　high　EQE　values　of　11.0%　and　5.5%　with　emission　peaks　at　477　and　467nm,　respectively.　These　achievements　are　enabled　by　a　vapour-assisted　crystallization　technique,　which　largely　mitigates　local　compositional　heterogeneity　and　ion　migration.　Achieving　bright　and　efficient　blue　emission　in　metal　halide　perovskite　light-emitting　diodes　has　proven　to　be　challenging.　Here,　the　authors　demonstrate　high　EQE　and　spectrally　stable　blue　light-emitting　diodes　based　on　mixed　halide　perovskites,　with　emission　from　490　to　451nm　by　using　a　vapour-assisted　crystallization　technique.