How long, and in what cases did Catholics participate in the Black Fast? I ran across the term "Black Fast" and looked it up.  The articles I'm finding indicate that it's extreme, and give some information about the types of foods allowed during it, but I keep expecting to find information on when it was performed and why.  In my experience, in Catholic tradition, there is a detailed reason and circumstance for just about everything, and I'm just not finding it.

Also, based on the restrictions, this looks to be something that is intended to be something done for an extended period of time, not just a few days...


  The details of the fast, as they were prior to the tenth century, are
  as follows:
  
  
  No more than one meal per day was permitted
  Flesh meat, eggs, butter, cheese and milk were forbidden
  The meal was not allowed until after sunset
  Alcohol was forbidden
  During Holy Week, the meal consisted exclusively of bread, salt, herbs, and water
  


No more than one meal per day would be hard for a few days, but doable, so I have to assume that this fast is meant for extended periods.

The closest I could come to a "how long" answer was a blurb in Wikipedia, but it it about how the Eastern Orthodox Church practices it today.


  The Black Fast is still observed by the Eastern Orthodox on Wednesdays
  and Fridays and during the 40 days of Lent and three other fasting
  periods of the year.


So, if anyone can fill in the blanks, and explain when it is done, why it is done, and for how long it is done, I'd greatly appreciate it.
You are right. Eastern Orthodox still practice this in a way, though it is not called the &quot;Black Fast&quot;. As I am an Orthodox Christian I can tell you that we do fast every Wednesday and Friday, though not as strictly as the Black Fast. It's more like eat vegan, basically the same as you listed, but we get 3 meals at the regular times, though some people may do it more strictly including monastics. Also don't eat as much.
Then we also have as we call it the Great Fast, or Great Lent, that starts 40 days before Holy Week. Basically we eat the same as on Wednesdays and Fridays, also trying more then normal to abstain from sin (that's the whole goal right?). Then during Holy Week the rules change and it is a little more strict, namely maybe 1 meal a day, on special days (Holy Friday: the crucifixion) eating nothing at all. Check out wikipedia for more info.
We also have fasts before the Dormition (two weeks long called the Dormition Fast), a fast called the Apostles Fast, and A fast before Christmas, called Advent. According to Wikipedia, Advent starts on the fourth Sunday before December 25, which is the Sunday between November 27 and December 3. According to my Orthodox Calendar, this year it will start on November 15.
Also I forgot to mention, that on the first week of Great Lent we have a strict fast from Monday to Wednesday, thats a strict fast meaning no food at all. For the weak, (Children, Elderly, Pregnant) there are exceptions.
Of course thats not all the fasts that we do. Check this link for more info.
OK, just checked back at your question and saw that you asked why it is done.
Um, kind of for the whole going to heaven thing that we're doing as Christians. Depriving the body of food is supposed to help with repentance and to humble the body to the Spirits will.
Hope that answers your question! If not I can say even more.