How can I use sips, imagemagic, or another tool to convert an image to black and white (grayscale) via the command line?
If you have imagemagick installed,
convert source.jpg -colorspace Gray destination.jpg (true grayscale only)
convert source.jpg -monochrome destination.jpg (true black and white)
convert source.jpg -separate destination.jpg (separate into gray channels)
If you don't care about losing the original file: mogrify -colorspace Gray file.