I can't say for certain how John managed the cool watermark trick, but I suspect he's using a WordPress plug-in that applies some CSS customization (in this case, a background image and a special border) to all of his comments.
If you're a WordPress user, there are a couple of plug-ins all custom CSS for author comments, though I couldn't say which one John is using. A quick glance at the description of
Highlight Author Comments by Rob Marsh leads me to believe it could be used to achieve a similar result.