The ASP.NET Membership API makes this very easy. All you have to do is have both a blog and forums using the same membership provider and connect to the same data store (most likely, to the SQL database), and both applications will use the same login and password.
However, if you want to provide a "single sign" so that they do not need to re-enter their password when switching from a forum to a blog, then you will want to implement both the blog and the forum functionality within the same site / application.
Regarding BlogEngine.NET; this is a really great starting point for building a website. A blog and a membership provider already exist, so all you have to do is add the functionality of the Forum.
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