How do I block someone who's been following my blog secretly? I couldn't see her in my Followers list (hence I couldn't use the "Block this user" link), but I have looked at her profile and could see that she's Following my blog.
Following, when you look at the bottom line, is no more than a feed subscription and an icon (possibly) displayed on your blog, and linking back to the profile of the Follower in question. If someone Follows your blog anonymously, all that they get is a subscription to the blog feed.
If you publish a feed from your blog, and if the feed is open to anybody (which, right now, is the case), then it's open to everybody. If someone wants to use Following to subscribe to the feed, you can't stop this. You can't block it before, or after, the fact. You can't Block what you can't see. Nor can you see an anonymous Follower.
Don't worry about what you can't see. Concentrate on what you can see, and on providing interesting and relevant content for your readers.