Why A solar flare releases a burst of X-rays and extreme UV that travels at the speed of light, reaching Earth in about 8 minutes. That radiation dumps extra ionization into the D layer on the sunlit side of Earth, and the D layer absorbs HF signals rather than refracting them. The result is a sudden ionospheric disturbance: HF, especially the lower bands, goes dead for minutes to a couple of hours until the D layer relaxes.
Watch out Coronal mass ejections are the tempting pick, but the plasma cloud from a CME takes roughly one to three days to reach Earth and produces geomagnetic storms, auroras and multi-day disturbed conditions rather than a sudden short blackout. A north-oriented interplanetary magnetic field actually shields Earth; it is a southward IMF that couples with the magnetosphere.
Flare = fast (8 minutes, short blackout). CME = crawls (days, long geomagnetic storm).