How long, and in what cases did Catholics participate in the Black Fast?
First of all let us start with a general Catholic definition of a black fast.
A rigorous form of fasting which, in its most classic form, was limited to one meal per day, not taken before evening, and excluding all flesh meat, eggs, butter, cheese, and milk. Variations have been in force in various times and places.
[Note: This has been changed from Fr. Hardon's original definition, which appears to have been incorrect: “Abstinence from all flesh meat, with only eggs, butter, cheese, or milk allowed at the evening meal. Formerly a diet limited to bread, salt, vegetables, and water constituted a black fast during Lent.” See the Catholic Encyclopedia.]
Prior to the changes in fasting by Pope Paul VI, the black fast was done by many of the faithful during the Season of Lent. I is still done in some very traditional religious communities that follow the Extraordinary Form of the Mass.
It was also done at ordinations in the Tridentine Rite. And I would not be surprised if it is still observed in those communities that celebrate the Mass according to Pope St. Pius V.
This form of fasting, the most rigorous in the history of church legislation, was marked by austerity regarding the quantity and quality of food permitted on fasting days as well as the time wherein such food might be legitimately taken.
In the first place more than one meal was strictly prohibited. At this meal flesh meat, eggs, butter, cheese, and milk were interdicted (Gregory I, Decretals IV, cap. vi; Trullan Synod, Canon 56). Besides these restrictions abstinence from wine, specially during Lent, was enjoined (Thomassin, Traité des jeûnes de l'Église, II, vii). Furthermore, during Holy Week the fare consisted of bread, salt, herbs, and water (Laymann, Theologia Moralis, Tr. VIII; De observatione jejuniorum, i). Finally, this meal was not allowed until sunset. St. Ambrose (De Elia et jejunio, sermo vii, in Psalm CXVIII), St. Chrysostom (Homil. iv in Genesim), St. Basil (Oratio i, De jejunio) furnish unequivocal testimony concerning the three characteristics of the black fast. The keynote of their teaching is sounded by St. Bernard (Sermo. iii, no. 1, De Quadragesima), when he says "hitherto we have fasted only until none" (3 p.m.) "whereas, now" (during Lent) "kings and princes, clergy and laity, rich and poor will fast until evening". It is quite certain that the days of Lent (Muller, Theologia Moralis, II, Lib. II, Tr. ii, sect. 165, no. 11) as well as those preceding ordination were marked by the black fast. This regime continued until the tenth century when the custom of taking the only meal of the day at three o'clock was introduced (Thomassin, loc. cit.). In the fourteenth century the hour of taking this meal was changed to noon-day (Muller, loc. cit.). Shortly afterwards the practice of taking a collation in the evening began to gain ground (Thomassin, op. cit., II, xi). Finally, the custom of taking a crust of bread and some coffee in the morning was introduced in the early part of the nineteenth century. During the past fifty years, owing to ever changing circumstances of time and place, the Church has gradually relaxed the severity of penitential requirements, so that now little more than a vestige of former rigour obtains. Black Fast (Catholic Encyclopedia
Some Eastern Catholics still practice the tradition of the black fast on Fridays during Lent, especially on Good Friday.
A priest who undertakes the office of exorcist should be himself a holy man, of a blameless life, intelligent, courageous, humble, and he should prepare for the work by special acts of devotion and mortification, particularly by prayer and a fasting (Matthew 17:20)
20 But this kind is not cast out but by prayer and fasting.
Exorcists take their work very seriously. Fr. Gabriele Amorth mentions that the black fact is common place among exorcists in his books, An Exorcist Tells His Story and An Exorcist: More Stories.
Although decline in this practice has been noticed within the Catholic Church, it is still practiced in some more traditional circles within Catholicism.