I am using
ffmpeg -i input.mp4 -vf "v360=input=fisheye:ih_fov=122.6:iv_fov=94.4:output=hequirect:h_fov=180:v_fov=180:in_stereo=2d:out_stereo=sbs,format=yuv420p" -b:v 25M -maxrate 60M -bufsize 25M output.mp4
Which works as advertised to convert a fisheye to hequirect except for two problems:
The background is not black but seems to try and set it to the average color of the video. It should be black throughout.
The corners are not blurred but should be, there should be a soft transition from the corner of the video into black.