How to detect whether an image is almost blank on Linux?

How to detect whether a .jpg image is almost blank/black on Linux?

Here is a bash function that I used to detect whether an image is black on Linux:

function isBlank ()
{
    mean=`convert $1 -format "%[mean]" info:`
    echo "$mean"
}

It uses ImageMagic to generate formatted image characteristics (the mean).

If the mean is 0, it is a black image.