And still is different from any rock 'rock' now.
As much as I love The Court of the Crimson King, there might be a good reason for that. Rock was going to some dark places around 1970.
Once upon a time, Blue Oyster Cult was the epitome of Prog Rock. Then the 80s happened, and everything original was run into the ground. Such is the cycle of creativity.
As much as I love them, BOC was never progressive per se. They did release a concept album, in the 80s even (well, pretty much anything written by Sandy Pearlman was part of a story in itself anyhow), but their songwriting was much more traditional. Back in the day, they were called Heavy Metal (but so was very very progressive Rush), and they described themselves as Psychedelic at least once.
Plus, it wasn't 80s' fault. By the late 70s many bands had been breaking up or jumping the shark. ELP tried to get on the disco bandwagon just as disco was about to get tackled and stomped into a thin red paste, VdGG lost half of the members and unsuccessfully experimented with New Wave, King Crimson was on hiatus from '74...
It was in the 80s (early, though) that Rush released two highly celebrated albums, on the other hand - they catched the synthpopitis later, but nonetheless.
And then, in 1987 a certain trio known as Myung, Petrucci and Portnoy met and went on to make a brain transplant of prog onto a body of metal, and in the same year a certain Steven Wilson did the same to alternative rock.
So, counting from post-Moving Pictures, the Dark Age of Prog lasted from 1983 to 1987. Of course, this is only counting the period where there was nothing at all and all hope was lost, and the plummet in general would be longer, but that gets much more blurred if you stray outside that.