Gemara

R. Yitzchak: Berachah comes only on something hidden from view - "...ha'Berachah ba'ASaMecha" (this is like SaMuy, hidden);

Tana d'vei R. Yishmael: Berachah comes only on what the eye does not rule over (cannot see) - ba'Asamecha (in your storehouse, which is hidden).

Beraisa: One who goes to measure his harvest says 'may it be Your will to send a blessing on the work of our hands;

While measuring, he says 'Baruch is He who sends Berachah to this pile';

If he blesses after measuring this is a vain prayer. This is because Berachah does not take effect on anything weighed, measured or counted, only on something that is not seen.

Chulin 105b: A Mazik broke a barrel; Mar bar Rav Ashi excommunicated the Mazik and demanded that it pay. Mar bar Rav Ashi agreed to give it time to pay.

The Mazik was late. It said 'we may not take from what is wrapped, sealed, measured or counted.'

Rishonim

Rif and Rosh (Berachos 44a and 9:18): One who goes to measure his harvest says 'may it be Your will to send a blessing on the work of our hands. After beginning to measure he says 'Baruch is He who sends Berachah to this pile'. If he blesses after measuring this is a vain prayer. This is because Berachah does not take effect on anything measured, weighed or counted, only on something hidden - "...Es ha'Berachah ba'Asamecha".

Divrei Chamudos (59): Rmy says that the Berachah is without Hash-m's name and Malchus. This is unlike Rashi (DH Baruch). Also the Rosh said at the beginning of our Perek (9:1) that all Berachos taught in our Perek are with Shem and Malchus, and he did not say that this is an exception.

Note: Perhaps 'taught in our Perek' refers to Berachos in the Mishnayos (or Gemara) of Perek 9 of Berachos, not what the Rosh himself cites from elsewhere!

Rambam (Hilchos Berachos 10:22): One who goes to measure his harvest first asks Hash-m to send a blessing on it. While measuring, he blesses to thank Hash-m for sending a Berachah. If he requests after measuring this is a vain prayer. Any request regarding something that already happened is a vain prayer.

Kesef Mishneh: The Ritva (Bava Metzi'a 42a) says that this is a proper Berachah with Shem and Malchus. The Ramban says that the Chiyuv Berachah is only for one who will separate Terumah and Ma'aseros, for Hash-m promised that he will be blessed - "Havi'u Es Kol ha'Ma'aser... va'Harikosi Lachem Berachah Ad Bli Dai". The Berachah was enacted to declare that Hash-m's promise is fulfilled.

Maharsha (Bava Metzi'a 42a DH ha'Holech): It is necessary to measure only for the sake of Ma'aser. The Berachah was enacted so that one will not lose through this.

Birkei Yosef (OC 230:6): The Berachah is a request for the future (that Hash-m send increase). If it were thanks, it would be appropriate even after measuring.

Question: Here we say that Berachah rests on what is not measured. In Chulin, it says that Mazikim may take only from such things!

Answer #1 (Tosfos DH Ela): Mazikim may take only from Hefker. If something (even if it has an owner) was not measured and increased due to the Berachah, it is like Hefker.

Meshech Chachmah (Haftoras Vayera): Elisha asked the widow (whose oil was miraculously increased) "Mah Yesh Lach". There is an extra 'Yud' in "Lach" allowing us to read this 'Li'. The Kelim of heirs are Meshubadim to the creditor, so the creditor's Kelim would acquire the increase due to the Berachah, which is Hefker. This would not be payment! Elisha asked if she had her own Kelim (which she could sell to him) and told her to borrow, so that the oil would pay the debt.

Answer #2 (Tosfos, ibid.): They may take from something measured only if it is not sealed.

Answer #3 (Prishah OC 230:5): Something that is not measured can be blessed if one prays for it. If one does not pray, it is prone to be diminished, and Mazikim can take from it

Me'iri (DH ha'Holech): He does not request a Berachah that the produce increase, rather, that he should prosper in his business.

R. Avraham Sofer (Hagahah 17): The Me'iri learns from the Yerushalmi, which says that we do not pray for miracles.

R. Bechayei (brought in Ein Yakov Bava Metzi'a 42a, DH ha'Holech): Very few people merit an overt miracle, i.e. a Berachah on something counted. Ayin ha'Ra rests on it. This is why Hash-m commanded to count Benei Yisrael indirectly, through Shekalim.

Maharsha (Bava Metzi'a 42a, DH b'Davar): Ayin ha'Ra prevents Berachah from resting on something exposed.

Rebuttal (Toras Chayim, brought in Ein Yakov ibid., DH Ela): There is no Berachah on something exposed, and not on one who measures his grain. Surely one does not put Ayin ha'Ra on his own grain! Rather, it is honorable for miracles to be covert. This is why Elisha told the woman to close the door before pouring.

Poskim

Shulchan Aruch (OC 230:2): One who goes to measure his harvest says 'may it be Your will to send a Berachah on this pile'. While measuring, he says 'Baruch is He who sends Berachah to this pile'. If he blesses after measuring this is a vain prayer, because Berachah comes only on something Samuy (hidden) from view.

Prishah (4): If he 'blesses', i.e. requests after measuring, this is a vain prayer.

Question: Why do the Tur and Shulchan Aruch say that the request is 'to send a Berachah this pile?' The Gemara, Rif, Rambam and Rosh say that we request Berachah on all deeds of our hands!

Answer #1 (Eliyahu Rabah, brought in Birkei Yosef 5, DH umi'Toch): The Tur and Shulchan Aruch discuss nowadays, when we do not take Ma'aser.

Answer #2 (Birkei Yosef): The Tur saw a different text, and favored it because the Tosefta supports it.

Birkei Yosef (Shiyurei Berachah 2): We request and bless when measuring one species, all the more so one when counts all his property before swearing about this regarding taxes! Or, perhaps we request and bless only regarding things that grow, for this is like an Aveidah (the seeds) which Hash-m returned with 100 times increase. One should bless without Shem and Malchus.

Acharonim

Question: What is the difference between the Drashos of R. Yitzchak and Tana d'vei R. Yishmael?

Answer #1 (Maharsha ibid., DH Ela): They argue only about from where we learn.

Answer #2 (ha'Rif in Ein Yakov, ibid.): Tana d'vei R. Yishmael holds that Berachah comes on anything whose quantity is unknown, even if it is visible. R. Yitzchak says that Berachah is only on something hidden from view.

Answer #3 (Iyun Yakov ibid., DH Ela): R. Yishmael holds that Berachah comes on anything whose quantity is now unknown. R. Yitzchak says that Berachah is only on something whose quantity was never known.

Answer #4 (Ben Yehoyada Bava Metzi'a DH Ein (2)): If one sees a bag (or mats) and can tell that there are coins inside (or grain underneath), the eye rules over it, but since the coins themselves are not seen, it is called 'Samuy'.

Question (Iyun Yakov): It says that Berachah is not Metzuyah (common) on something measured. This implies that it is possible. If so, why is the Tefilah vain?