In some browsers at some resolutions there appears to be a faint 1px line around background block media.

This is likely some kind of anti-aliasing rounding error that lets part of the background peak through around the image.
A hacky fix is to scale the <img element, but this might have other implications (for example, adjacent images that are intended to line up perfectly no longer doing so) and 1% could be a pretty large change for some images. For the article I was testing with a much smaller 0.2% scale still works.
