We designed and synthesized a new organic light-emitting diode (OLED) emitter, SBABz4, containing spiro-biacridine donor (D) in the core surrounded by two benzonitrile acceptors (A). The dual A-DxD-A structure is shown to provide pure-blue emission in relation to its single A-D counterpart. Time-resolved photoluminescence (TRPL) recorded in the broad dynamic range from solutions and solid films revealed three emission components: prompt fluorescence, phosphorescence, and efficient thermally activated delayed fluorescence (TADF). The last is independently proven by temperature-dependent TRPL and oxygen-quenching PL experiment. From the PL lifetimes and quantum yield, we estimated maximum external quantum efficiency of 7.1% in SBABz4-based OLEDs and demonstrated 6.8% in a working device.