If a person accepts upon himself a Ta'anit Yahid – a personal fast day – he is bound by this vow and may not eat on the designated day. If he violates his vow and partakes of food or drink on the designated fast day, does he recite a Beracha?
On regular fast days, such as Yom Kippur, a person who eats in violation of Halacha does not recite a Beracha before eating. (If one must eat on a fast day due to a health condition, then Halacha permits eating and thus he of course recites a Beracha.) Seemingly, then, we would conclude that on a private fast day, too, one who eats in violation of Halacha should not recite a Beracha.
Rabbi Moshe Halevi (Israel, 1961-2001), however, in his work Birkat Hashem (vol. 2, chapter 1; listen to audio for precise citation), distinguishes between ordinary fast days and fasts observed by force of personal acceptance. On fasts such as Yom Kippur, eating is forbidden intrinsically, as evidenced by the fact that all Jews are forbidden to eat on these days. In the case of a private fast, however, eating is not intrinsically forbidden. One who eats in such a case has not committed a forbidden act of eating, but has rather violated his vow. His violation relates not the eating itself, but rather to the result of this eating upon the vow that he had declared. As such, a Beracha is warranted despite the Halachic violation entailed. Since the violation does not relate directly to the act of eating in this case, the individual would, in fact, recite a Beracha.
Does one recite a Beracha if he stole food which he now wishes to eat?
Halacha clearly forbids partaking of stolen food, and thus stolen food has the formal Halachic status of "Ma'achalot Asurot" (forbidden food). Hence, as Rabbi Moshe Halevi rules, just as one who eats non-kosher food does not recite a Beracha before eating, similarly, a thief who eats the stolen goods would not recite a Beracha. Since the food is intrinsically forbidden, it does not warrant the recitation of a Beracha.
Rabbi Moshe Halevi applies this rule also to the case of one who eats food deemed "Mukseh" on Shabbat. For example, a fruit that falls from a tree during Shabbat is considered "Mukseh" and may not be eaten or handled on Shabbat. The status of such fruit on Shabbat thus resembles that of non-kosher food, and hence a person who partakes of this fruit on Shabbat would not recite a Beracha. Likewise, if a person cooks food on Shabbat in violation of Halacha, the food is forbidden for consumption until after Shabbat (unless it was cooked for a direly ill patient in a situation where Halacha permits cooking). Therefore, a person who eats this food on Shabbat does not recite a Beracha, as he partakes of food that is intrinsically forbidden for consumption.
Summary: If a person accepts upon himself a voluntary fast and eats in violation of his vow, he nevertheless recites a Beracha over the food. However, if somebody eats stolen food, or food that was cooked on Shabbat or is otherwise considered "Mukseh" on Shabbat, he does not recite a Beracha over such food.
