Considering the instability and low photoluminescence quantum yield (PLQY) of blue‐emitting perovskites, it is still challenging and attractive to construct single crystalline hybrid lead halides with highly stable and efficient blue light emission. Herein, by rationally introducing d10 transition metal into single lead halide as new structural building unit and optical emitting center, we prepared a bimetallic halide of (NH4)2CuPbBr5 with new type of three‐dimensional (3D) anionic framework. (NH4)2CuPbBr5 exhibits strong band‐edge blue emission (441 nm) with a high PLQY of 32 % upon excitation with UV light. Detailed photophysical studies indicate (NH4)2CuPbBr5 also displays broadband red light emissions derived from self‐trapped states. Furthermore, the 3D framework features high structural and optical stabilities at extreme environments during at least three years. To our best knowledge, this work represents the first 3D non‐perovskite bimetallic halide with highly efficient and stable blue light emission. It turns blue! The first crystalline three‐dimensional bimetallic halide of (NH4)2CuPbBr5 exhibits strong narrow blue emission with high PLQY of 32 % as well as remarkable structural and optical stabilities. This structural design strategy is entirely different from the modification methods over CsPbX3 quantum dots and paves a unique way to realize blue light emission based on single‐crystalline hybrid metal halides.