A basic principle in the laws of Berachot establishes that one does not recite a Beracha over food if partaking of the food constitutes a Halachic violation. Thus, for example, a person who eats on a fast day in violation of Halacha does not recite a Beracha before eating (unless he must eat for medical reasons, in which case Halacha permits eating, and he therefore recites a Beracha). This is the ruling of Rabbi Moshe Halevi (Israel, 1961-2001), in his work Birkat Hashem (vol. 2, p. 77; listen to audio for precise citation).
This principle gives rise to a number of interesting questions involving other situations in which eating is forbidden. For example, Halacha forbids eating or drinking on Shabbat before one recites Kiddush, as well as on Mosa'e Shabbat before reciting Havdala. The question thus arises, if a person eats before reciting Kiddush on Shabbat or before reciting Havdala after Shabbat, in violation of Halacha, does he recite a Beracha over this food? At first glance, it would appear that here, too, a Beracha is not warranted given that the eating is performed in violation of Halacha.
However, Rabbi Moshe Halevi (listen to audio for precise citation) draws an important distinction between these cases and the aforementioned case of one who eats on a fast day. Eating before one recites Kiddush or Havdala is not forbidden intrinsically, but rather as a measure to ensure proper respect for the Kiddush or Havdala. When one eats or drinks before Kiddush, his violation relates not to the act of eating itself, but rather to the effect of this eating on Kiddush, namely, that he has not afforded the proper respect to Kiddush. Hence, since the eating does not intrinsically constitute an act of Halachic violation, one would recite a Beracha in such a case. Of course, this would apply as well to somebody who eats before reciting Havdala.
This rule would similarly apply in a case where one eats before reciting the Shaharit prayer in the morning. The Sages forbade eating before praying because praying on a full stomach is an expression of Ga'ava (arrogance). Here, too, the act of eating is not forbidden intrinsically, but rather due to its impact upon the prayer recited thereafter. Hence, in this instance, as well, one who eats in violation of Halacha would nevertheless recite a Beracha.
Summary: One who eats on a fast day does not recite a Beracha before eating, unless he is allowed to eat due to a serious health condition. One who eats on Shabbat before reciting Kiddush, after Shabbat before reciting Havdala, or in the morning before praying Shaharit, recites a Beracha despite the fact that his eating constitutes a Halachic violation.
