During the late 90's - early 2000's they used to be printed in the little paper folder the pass came in, and were what you would expect. Major holidays. Prior to that, I guess they didn't exist. This blog has an image with a couple different years. You can click for a larger image to read the ones for 1998 and 2000.
http://disneyonparole.blogspot.com/2009/06/disney-main-gate-pass-and-silver-pass.html
So in 1998, MK was Blacked out a total of 30 days: Jan 1/2, April 5-18, July 1-7, Dec 24-31.
In 2000, they blacked out MK 8 days: July 4th & Dec 24-31.
Looks like a decrease. Even those 30 days in 1998 seem reasonable compared to today.