I have an unpublished draft post saved to my Blogger account. Being "unpublished", you would think that no-one else can read it yet. Surprise! You can view it here. As you can see, I even posted a comment to it. Bizarre. It's still unpublished, at least. (That is, the item page does not exist.) But the content is publicly accessible, which Blogger drafts presumably shouldn't be. So the upshot is similar enough.
Blogger users take note: anyone can read your drafts if they learn the postID (and hence can load the comment window using a similar link to my above example).
Of course, it'd probably never happen in practice, since (1) how would a would-be spy find the right postID? and (2) who would want to read our drafts anyway?
Still, as a matter of principle, Blogger probably shouldn't be leaving our saved drafts out in public cyberspace like that, for anyone to stumble upon...