Prior to the Middle ages Ashes where a sign of mourning and repentance, going back in the middle east among the Jews as a rite. The term Sackcloth and Ashes comes from the practice of covering oneself with ashes or dirt, (dust of the earth).
Up until recently the ashes in Catholic practice where imposed on the top of the head. Women’s hats where a primary reason for moving the ashes to the front of the head. Until the early 1970’s when men who where preparing for the priesthood recieved ashes they still recieved them on the tops of the head, in the area where the bishop or religious superior had cut the tonsure. Tonsure was the rite in which the hair was cut in five (in some places four) locks that represented the five wounds of Christ, in the form of the cross, and indicated the entrance into the clerical state. The Monastic Tonsure was where a belt of other suitable item was placed around the head, and what was not covered by the belt was shaved, (Thomas Aquinas, St. Dominic, Saint Anthony and St. Francis are usually depicted with the Monastic Tonsure) So you can see that sprinkling the head of a Tonsured Cleric would not hide the ashes, but rather be a profound symbol of ones mourning over ones sins.
Up to the middle ages, public sinners would present themselves at the Cathedral Church in sackcloth, have the ashes imposed and then go to a monasary or convent to do penance for the 40 days of Lent. In the Middle ages the ashes where imposed on anyone who wished to amend their lives, so while it is correct to say that the widespread use of ashes developed in the Middle Ages, the practice comes down to us from the pre-Christian Jewish custom, and has been revered by the Church since the days of the Apostles.