I think it would help to see what's new. I don't have much to add, it's a simple feature that IMO would make things look a little better. The image explains the feature request.
This has been deployed now.
Note that if an item has been "collapsed" (like comments are) then if even 1 item in the collapsed set is new it will be highlighted.
Basically, the # of new messages is not necessarily equal to the # of highlighted blocks.