Organic light-emitting diodes (OLEDs) are a promising light-source technology for future generations of display1,2. Despite great progress3–12, it is still challenging to produce blue OLEDs with sufficient colour purity, lifetime and efficiency for applications. Here, we report pure-blue (Commission Internationale de l’ Eclairage (CIE) coordinates of 0.13, 0.16) OLEDs with high efficiency (external quantum efficiency of 32 per cent at 1,000 cd m−2), narrow emission (full-width at half-maximum of 19 nm) and good stability (95% of the initial luminacnce (LT95) of 18 hours at an initial luminance of 1,000 cd m−2). The design is based on a two-unit stacked tandem hyperfluorescence OLED with improved singlet-excited-state energy transfer from a sky-blue assistant dopant exhibiting thermally activated delayed fluorescence (TADF) called hetero-donor-type TADF(HDT-1) to a pure-blue emitter. With stricter control of device fabrication and procedures it is expected that device lifetimes will further improve to rival commercial fluorescent blue OLEDs.Pure-blue organic LEDs with narrow emission and improved stability show promise for display applications.