Block quotes are a good (if semantically questionable) way to visually separate e.g. screenshots as part of a post on SU, which might otherwise be mistaken as part of the post text, since the get a line on the left side (similar to some email clients' quoting).
On MSU, where we regularly include e.g. screenshots of the regular site into posts, it's different:
Unfortunately, it's not nearly as useful when including partial screenshots of a web page, this the background color becomes a not nearly as visible border around the image. Compare the following screenshots taken from a slightly modified version of this post by slhck.
Block quote MSU
Block quote SU
The border on the left side works much better to separate the images from the text of the post.
Please chance the MSU
blockquote style to the one used on SU (maybe with less/no saturation in the border color, but a similar overall style).