No URLs need apply. I am thinking that in BlogBridge the standard way to sign up to a feed should be to use a search.
In other words, "find me a blog about ..." which would produce a list of blogs, perhaps in relevance order, so that the user can choose which blog or feed he'd like to follow.
No URLs need to be typed by the user. (Of course there will be an advanced feature somewhere if you insist.) This idea comes from hearing rants over the years about how URLs and DNS names are too primitive and get broken over time, whereas Google has become the de-facto tool to navigate to a web site, at least in all but the most obvious cases.
By analogy, this question of whether the user types in the URL of the feed or the URL of the blog may be looking at things the wrong way. The user really shouldn't have to locate any URLs. If you are looking for a blog about X, just ask for it.