A novel approach to highly efficient, blue thermally activated delayed fluorescent (TADF) emitters is proposed. A series of ortho-carbazole-appended triarylboron compounds (1-5) are prepared with various substituents, including tert-Bu, Me, and OMe, introduced to the carbazole donor and/or to the dimesitylphenylboron (PhBMes(2)) acceptor. Depending on the substituents on the donor and/or acceptor moieties, the emission color is finely tuned over the entire blue region from sky blue to ultradeep blue. Retention of a twisted donor (D)-acceptor (A) structure enabled by the ortho D-A connectivity and bulky triarylboron leads to small energy splitting (Delta E-ST) between singlet and triplet excited states, resulting in very efficient TADF. High-efficiency blue TADF organic light-emitting diodes (OLEDs) are realized by using the proposed ortho D-A compounds as emitters. Among them, the blue OLEDs incorporating a BuCzMeoB (2) emitter exhibit a very high external quantum efficiency (EQE) of 32.8% at Commission Internationale de l'Eclairage (CIE) color coordinates of (0.135, 0.266). The high device efficiency is attributed to both the high photoluminescence quantum yield of 2 in a given host film (93%) and the high horizontal transition dipole ratio (Theta) of 0.76. Ultradeep blue OLEDs with a record-high efficiency are also fabricated with a CzOMeoB (5) emitter, exhibiting an EQE of 14.9% at CIE coordinates of (0.151, 0.058).