It's not a pop-up Canyon, because it doesn't come up in a separate window. That's why your blocker can't get it. It's a regular, in-line ad that expands its bounds (by way of Flash player, as dbab mentioned). I get the same sort of ads on MSN on occasion and they drive me batty too. At least this ad had a large "close X" box. Most have a teeny-tiny one that's hard to find and that usually leads you to accidently clinking through on the link, instead of closing it. That's why they do it. Some even take up the whole screen.
While ads are great for keeping services free, they are a double-edged sword. I quit checking in here yesterday evening because I didn't want to have to keep turning the ad off everytime I looked at the main board, an individual message, or made a reply. So they can drive people away.