COMMAND:"
ffmpeg -i art.jpg -filter_complex "[0:v]scale=1920:ih,setdar=16/9,boxblur=10[bg];[bg][0:v]overlay=(W-w)/2:(H-h)/2[v]" -map "[v]" -map 0:a -c:a copy output.jpg

Output:
Stream map '0:a' matches no streams.