Blue organic light-emitting diodes (OLEDs) utilizing triplet-harvesting emitters require the use of high-triplet-energy hosts and blocking layers to confine the triplet excitons to the emitter. The use of these materials poses design challenges, while potentially compromising charge transport and operational stability. Here, we present efficient single-layer blue OLEDs comprising solely of a neat thermally activated delayed fluorescence (TADF) emitter sandwiched between two charge-injecting electrodes, without using high-triplet-energy materials. By further incorporating a narrow-band terminal emitter, we simultaneously improve the charge balance and color purity, realizing pure-blue single-layer hyperfluorescent OLEDs with an external quantum efficiency (EQE) of 21.1% and minimal efficiency roll-off. Analysis of the charge transport reveals that the improvement in charge balance is caused by the offset in ionization energy between the TADF sensitizer and the terminal emitter, slowing down hole transport. Our results demonstrate the feasibility of efficient pure-blue single-layer OLEDs without auxiliary high-triplet-energy materials, featuring a simple design and added stability benefits.