Is Pink Floyd prog rock?

Why do I often read opinions like this one that seem to find the blues necessarily not progressive or less metal?  When Priest made Stained Class, it was filled with the blues! It just found some other great modes and moods to fit in as well so the music wasn't as purely derived from the blues as it had been on Sad Wings. There's also plenty of blues in ELP, who have lots of fun jazzy passages which rely heavily on the blues. The blues isn't something that hold music back from progression, it's just another element to use in your playing and writing. Pink Floyd are perhaps the greatest example of how versatile and progressive the blues can be, honestly.

Stage musicals has been doing that for centuries for costume or scene changes. There is nothing "prog" about it -- in fact it is far more Broadway than prog. They even have words for it in several languages, like "intermezzo", "verwandlungsmusik", "entr'acte" or "incidental music", or for more specific play-within-play moments, divertimento or divertissement, or for quicker transitions "vamping", as in "vamp till cue".