Novel hole blocking materials (HBMs) based on dibenzo[b,d]furan and dibenzo[b,d]thiophene molecular dimers have been rationally designed and synthesized for high-efficiency and long-lived blue phosphorescent organic light-emitting diodes (PhOLEDs). Thermal, optical, and electrochemical analyses show that [2,2′-bidibenzo[b,d]furan]-6,6′-diylbis(diphenylphosphine oxide) (DBF-d-PO), [2,2′-bidibenzo[b,d]thiophene]-6,6′-diylbis(diphenylphosphine oxide) (DBT-d-PO), 6,6′-di(pyridine-3-yl)-2,2′-bidibenzo[b,d]furan (DBF-d-Py), and 6,6′-di(pyridine-3-yl)-2,2′-bidibenzo[b,d]thiophene (DBT-d-Py) possess high thermal stability, deep highest occupied molecular orbital energy levels (−6.61 to −6.95 eV), and high triplet energy (E_T) (2.68–2.95 eV). Blue PhOLEDs with DBF-d-PO, DBT-d-PO, DBF-d-Py, and DBT-d-Py exhibit low turn-on and operating voltages, excellent external quantum efficiency, and high current and power efficiencies. A blue PhOLED with DBF-d-Py shows the best efficiency with a maximum external quantum efficiency of 24.3%, a maximum current efficiency of 44.3 cd A⁻¹, and a maximum power efficiency of 46.4 lm W⁻¹. In addition, it exhibits an outstanding external quantum efficiency of 22.4% at a practical luminance of 1000 cd m⁻² and a very high maximum luminance of 88 953 cd m⁻² at 12 V. Furthermore, blue PhOLEDs with DBF-d-Py and DBT-d-Py exhibit highly improved lifetimes compared with the conventional HBM, BmPyPB, because of the efficient hole blocking by the deep HOMO energy level and the high thermal stability stabilizing hole blocking layer morphology.