The Shulchan Aruch writes (453:4) that the Matzot one uses to fulfill the obligation to eat Matza at the Pesach Seder should be produced from wheat that had been carefully watched "Mi'she'at Ketzira," from the time it was harvested. This means that from the moment the wheat was harvested from the ground somebody had guarded it to ensure that it did not come in contact with water, which would cause it to become Hametz. Matza produced from this kind of wheat, that had been carefully watched since it was harvested, is called "Matza Shemura" ("guarded Matza"), or "Matza Shemura Mi'she'at Ketzira."
The Peri Chadash (Rabbi Chizkiya Da Silva, 1659-1698) was of the opinion that this provision is "Me'akeiv," meaning, indispensable for the fulfillment of the Mitzva. If wheat had not been carefully guarded since the moment it was harvested, then one cannot fulfill his obligation at the Seder with Matza produced from this wheat, even if we have no reason to suspect that it had come in contact with water. Indeed, the Chid"a (Rabbi Chayim Yosef David Azulai, 1724-1806), in his work Machazik Beracha (460:3), records the ancient practice observed in Jerusalem to insist upon Matza Shemura for the Seder, adding that most respected communities likewise followed this practice.
Nowadays, Matza Shemura is readily available and one should therefore make a point of purchasing specifically this kind of Matza for fulfilling the obligation to eat Matza on the first two nights on Pesach.
Nevertheless, the Shulchan Aruch held that one who does not have Matza Shemura may fulfill his obligation with regular Matza. Preferably, he rules, one who does not have access to Matza Shemura should use Matza made from grain that had been guarded from the time it was ground. If even such Matza is unavailable, then one may even purchase ordinary flour from the market and use Matza produced from that flour. This is, indeed, the accepted Halacha.
In such a case, when one has access only to Matza that is not Shemura, should he recite the Beracha of "Al Achilat Matza" before eating this Matza?
Instinctively, we might apply here the famous principle of "Safeik Berachot Le'hakel," that we do not recite a Beracha in a situation where its obligation is subject to uncertainty. As we saw, the Peri Chadash held that one does not fulfill his obligation of Matza at the Seder with Matza that is not Shemura, and hence a Beracha recited over this Matza would constitute a Beracha Le'vatala ("wasted Beracha"). Seemingly, then, even though the Shulchan Aruch does not follow this view, we should require omitting the Beracha in such a case given the controversy surrounding this issue.
Chacham Ovadia Yosef, however, in his work Chazon Ovadia (Laws of Pesach, p. 76), rules that one indeed recites the Beracha of "Al Achilat Matza" when eating Matza that is not Shemura. He explains that the rule of "Safeik Berachot Le'hakel" applies only when the authorities disagree directly regarding the recitation of the given Beracha. Here, however, the dispute relates not to the issue of the Beracha, but rather to the question of whether Matza that is not Shemura is suitable for the performance of this Mitzva. The question surrounding the Beracha is merely a natural outgrowth of this debate. Therefore, since the Shulchan Aruch rules that such Matza indeed qualifies for use for the Mitzva, one who uses Matza that is not Shemura indeed recites a Beracha, in accordance with the Shulchan Aruch's ruling.
A similar example concerns the famous debate between Rashi and Rabbenu Tam regarding the proper sequence for writing the text inside the Tefillin. Even though this issue is subject to a debate among the authorities, we nevertheless recite a Beracha when wearing Rashi Tefillin, despite the fact that according to Rabbenu Tam we do not fulfill the Mitzva with such Tefillin. Once again, the debate among the authorities relates to the issue of whether a given item qualifies for the performance of the Mitzva, and does not directly address the question of the Beracha. Therefore, since the Shulchan Aruch accepts Rashi's position, we wear Rashi Tefillin and recite the Beracha, despite the controversy surrounding this issue.
Summary: It is preferable to use for the Mitzva of eating Matza on the first two nights of Pesach Matza made from wheat that had been carefully watched since being harvested. Nowadays, such Matza is readily available and one should therefore purchase specifically this kind of Matza. Nevertheless, one who does not have access to this kind of Matza may use Matza made from regular wheat, and may even recite the Beracha of "Al Achilat Matza."
