you should have a separate 'signature' for each purpose. When you are writing a work related email (for your employer) you use the work one with your tile (Joe Smith, Product Engineer, XYZ Corp). When you are responding to something about your blog or your side business, you use that signature (Joe Smith Joe's Web Design, IntellectualJerkoff.blogspot.com)
Your company guidelines might not say anything about your email signature, but I'd guess that if you start using your side business name in your signature, someone is going to have an issue with it and complain (rightly) that you are using XYZ Corp. resources to promote your own side business, and I doubt that would go over well with XYZ Corp.