I considered whether Blathertwaddle is a portmanteau word (before I got to your discussion of it, BTW) and discarded it. A true portmanteau word is a little more subtle.

Worthington Bank, for example. It is a bank that has offices in Fort Worth and Arlington. Worth-ington. It has nothing to do with the Worthington Hotel in downtown Fort Worth, and I don't know if that is a name or a portmanteau. Anyway, I don't think you can just stick to words together for a portmanteau, you have to lop a little off surgically and still have a pronounceable something left over.