Perovskite light-emitting diodes (LEDs) show promises for next-generation displays thanks to their excellent luminescent properties and low cost. Despite substantial progress with green- and red-emitting devices, the development of efficient blue perovskite LEDs has lagged behind. Here, we demonstrate efficient blue perovskite LEDs based on a mixed 2D-3D perovskite and a multifunctional ionic additive that enables control over the reduced-dimensional phases, non-radiative recombination channels and spectral stability. We report a series of devices that emit efficient electroluminescence from mixed bromide/chloride quasi-3D regions, with external quantum efficiencies (EQEs) up to 21.4% (at a luminance of 22 cd m-2 and emission peak of 483 nm), 13.2% (at a luminance of 2.0 cd m-2 and emission peak of 473 nm) and 7.3% (at a luminance of 6 cd m-2 and emission peak of 464 nm). Devices show a nearly thirtyfold increase in operational stability compared to control LEDs, with a half69 lifetime of 129 minutes at an initial luminance of 100 cd m-2. Our findings demonstrate performance of blue perovskite LEDs close to that of state-of-the art blue organic LEDs and inorganic quantum-dot LEDs, while providing a new approach to design multifunctional molecules to boost the performance of perovskite optoelectronic devices.