Before we look at Blerp, a reminder: Fleck and Diigo are two similar services we've covered; both have been around for quite a while, but haven't been able to achieve significant growth.
Blerp has the right idea; it puts emphasis on existing social networks, but doesn't really take the concept far enough. When you search for a site inside Blerp, it opens within Blerp's frame; you can explore and add notes, images and videos from the toolbar on the left side, as well as immediately post them to a number of other services: Facebook, Digg, StumbleUpon, Twitter, etc. However, there is no real integration with these services; you can only post links to them. Furthermore, the toolbar, which should be the central part of Blerp, has a flaw: it's crowded, and it takes a lot of scrolling to follow the discussion.
It also has a number of interesting options. For example, if you blerp a lot, you're becoming a top user, which are displayed in a special section on the site. The notes - blerps - can be positioned anywhere on the site, which gives them better context. However, the many options sometimes get in the way of the entire experience, which should be as simple as possible given that adding another information layer over web sites can be confusing in itself.
The idea behind Blerp, according to its creators, is to "liberate the web" and try to create a giant social forum, where people would discuss and enrich web destinations as they browse. The problem with this approach, however, is that people don't particularly like additional layers over web sites, for various reasons, speed and inevitable errors being the two most common ones. Just like with other similar sites, Blerp would work great if it were able to create a large user base, but it's easier said than done. The service is currently in "open alpha" stage, which means they're planning on tweaking it a lot; we'll see where the creators take it in the future.