Organic light-emitting diodes (OLEDs) play an important role in the new generation of flat-panel displays. Conventional OLEDs employing fluorescent materials together with triplet–triplet annihilation suffer from a relatively low internal quantum efficiency (IQE) of ~62.5%. On the other hand, the IQE of OLEDs employing phosphorescent or thermally activated delayed fluorescence (TADF) materials can reach ~100%. However, these materials exhibit very broad peaks with a full-width at half-maximum (FWHM) of 70–100 nm and cannot satisfy the color-purity requirements for displays. Therefore, the latest commercial OLED displays employ blue fluorescent materials with a relatively low IQE, and efficient blue emitters with a small FWHM are highly needed. In our manuscript, we present organic molecules that exhibit ultrapure blue fluorescence based on TADF. These molecules consist of three benzene rings connected by one boron and two nitrogen atoms, which establish a rigid polycyclic framework and significant localization of the highest occupied and lowest unoccupied molecular orbitals by a multiple resonance effect. An OLED device based on the new emitter exhibits ultrapure blue emission at 467 nm with an FWHM of 28 nm, Commission Internationale de l’Eclairage (CIE) coordinates of (0.12, 0.13), and an IQE of ~100%, which represent record-setting performance for blue OLED devices.