Gemara

(Beraisa): If one vowed from Peros ha'Aretz, all that grows from the land is forbidden. Mushrooms and truffles are permitted;

If he said 'what grows from the ground', all of them are forbidden.

Contradiction (Mishnah): The Berachah on something that does not grow from the ground is 'sheha'Kol';

A Beraisa teaches that one blesses sheha'Kol on salt, brine, mushrooms and truffles!

Answer (Abaye): Mushrooms and truffles grow from the ground, but they nurture from the air, not from the ground.

Question: The Mishnah discusses what does not grow from the ground!

Answer: It means, what does not nurture from the ground.

Berachos 36a (Rav Yehudah): One blesses Borei Pri ha'Adamah on wheat flour;

(Rav Nachman): One blesses sheha'Kol Niheyeh bi'Devaro.

Support (Rava, for Rav Yehudah - Shmuel and R. Yochanan): One blesses Borei Pri ha'Etz on olive oil.

Even though it changed, its blessing is the same. The same applies to flour!

Rejection (Rav Nachman): No. There, oil is the best product one can make from olives. One can make something better from wheat, namely, bread!

Question: Even if something better can be made from a food, this does not demote its blessing from Borei Pri ha'Adamah to sheha'Kol Niheyeh bi'Devaro!

(Shmuel): One blesses sheha'Kol (Niheyeh bi'Devaro) on raw gourds and on barley flour.

Inference: One blesses Borei Pri ha'Adamah on wheat flour!

Answer: No, one blesses sheha'Kol on wheat flour. Had he taught about wheat flour, one might have thought that one does not bless at all for barley flour.

Question: Why would one think that one does not bless? A Beraisa teaches that one blesses sheha'Kol even for salt or brine!

Answer: One might have thought that one blesses for those, for one sometimes puts them in his mouth, but not for barley flour, for one would not put it in his mouth, for this causes intestinal sickness. Shmuel teaches that this is not so.

Rishonim

Rif and Rosh (Berachos 25a): Presumably, the Halachah follows Rav Nachman who says that one blesses sheha'Kol on wheat flour, for people do not normally eat it. The same applies to barley flour. Shmuel taught that one blesses sheha'Kol on raw gourds and on barley flour.

Rosh (Berachos 6:2): We do not discuss finely ground flour from which we make bread. All agree that we do not bless Borei Peri ha'Adamah on it, for people do not eat it at all. It is unlike raw wheat or rice, which can be eaten raw. Rather, we discuss flour that was not ground so finely, and it still has some taste of wheat and can be eaten. Alternatively, it was totally ground from parched grain. Such flour is fitting to eat. The Rif rules like Rav Nachman, for people do not normally eat flour. Some say that since Rava said that he should not argue with Rav Yehudah, and Shmuel and R. Yochanan agree, the Halachah follows Rav Yehudah. Even though Rav Nachman defended himself, Rava did not accept this. Also, Rav Nachman did not say that we bless sheha'Kol because it is not normal to eat. If so, he should have answered that oil is different, for it is normal to eat Anigron (a mixture in which oil is the main ingredient)!

R. Yonah (DH v'Rav and DH Nimtza): The Halachah follows Rava, for he is Basra. Also, Rav Nachman did not bring a Mishnah or Beraisa, only reasoning. We rely on Rava's reasoning. The Ri ha'Zaken holds that they discuss flour of parched grain. If so, all agree that one blesses sheha'Kol on regular flour!

Rosh (ibid.): Why did the Rif rule like Rav Nachman for a reason that Rav Nachman disagrees with? Perhaps the Rif himself was concerned for this, for surely it is not normally eaten. According to this, one would bless Borei Peri ha'Adamah on flour of parched grain, since it is normally eaten. Since there is a an argument, one should bless sheha'Kol.

Ma'adanei Yom Tov (50): Even though Rav Nachman and Rav Yehudah are not concerned that it be normally eaten, there are other sources for this, e.g. Beitzah 16a. (Bishul Akum applies to fish in flour, because flour is not eaten raw.) Also, perhaps Rav Nachman gave an additional answer.

Rosh: According to this, one would bless Borei Peri ha'Adamah on flour of parched grain, since it is normally eaten. Since there is an argument, one should bless sheha'Kol.

Ma'adanei Yom Tov (60): I.e. perhaps the Rif really rules like Rav Yehudah, who is not concerned whether or not a better food could be made form this, as long as it is normally eaten.

Rif and Rosh (Berachos 28b and 6:24): One blesses sheha'Kol on salt, brine, and mushrooms and truffles.

R. Yonah (DH Gemara Al): Truffles grow underground, from fatness of the earth. Mushrooms grow on trees.

Magen Avraham (OC 204:4): Mushrooms and truffles grow from the air.

Rambam (Hilchos Berachos 3:2): One blesses sheha'Kol on flour.

Rambam (8:8): One blesses sheha'Kol on salt, mushrooms and truffles.

Poskim

Shulchan Aruch (OC 204:1): We bless sheha'Kol on the following... salt, saltwater, mushrooms and truffles...and barley flour.

Mishnah Berurah (5): One blesses on salt because he gets some pleasure when he puts it in his mouth.

Kaf ha'Chayim (6): We discuss salt that is fitting to it, i.e. it is not too bitter or moldy, and diluted saltwater that is fitting to drink. If not, one does not bless on them at all.

Magen Avraham (8): We bless sheha'Kol on barley flour because it changed, but did not reach the way it is eaten.

Shulchan Aruch (208:5): We bless sheha'Kol on flour, even of wheat. This is whether it was ground very finely, or only a little, and still has some taste of wheat, or whether it is of parched grain.

Kaf ha'Chayim (33): The Pri Chodosh says that it is proper to eat it only in a meal (to avoid the Safek), just like whole grains (Sa'if 4).

Kaf ha'Chayim (35): Some say that if one blessed Borei Peri ha'Adamah on flour of parched grain, he was Yotzei. I say that this applies to all flour, since the Berachah is true.