This is a pretty interesting article, along with this article on the economic limitations of variable renewable energy (e.g. solar and wind):
A half century ago, under bright moonlight of rush hour, they were playing the popular hit song, Jonathan King's Everyone is Going to the Moon -- as Dan Ingram would describe -- in a Key of "R", because the tape deck playing the song would slow down as the electrical frequency dropped at the start of the Great Northeast Blackout.
Definitely one of the great moments in radio history. A creepy song to be playing at the start of the blackout, especially under a full moon. Not to mention Jonathan King is a creepy individual -- he would later go to prison as a pedophile.
As they said, the electricity slowed down, as the load on the grid exceeded what power plants could put out, as they started shutting down automatically as they lost power and circuit breakers overloaded. The great power failure happened when the grid was most strained -- it was rush hour, on a cold night with lots of electric heat operating, along with street lights on and office buildings still lit brightly.
They say based on comparisons of this tape versus the normal speed recordings of Edward's Everybody is Going to the Moon, the grid frequency was about 50 Hz -- or the power had browned out to about 1/6th of the normal power. In an analog world, this meant lights dimmed, elevators ran slow, and tape decks played slowly -- until the power finally quit.
Here is the normal speed version of this song: https://www.youtube.com/watch?v=00XbDRuI78Y