Motel and sitcom are portmanteaus -- they're some of the most common examples used when defining the term "portmanteau" .

"Blend" and "shortening" are necessary-but-not-sufficient synonyms for portmanteau. Your error here is that you imply "because it's a blend, it's not a portmanteau." All portmanteaus are blends. All portmanteaus are shortenings. Some blends are portmanteaus. Some shortenings are portmanteaus.