Genesis 27:28

Therefore the Word of the Lord give thee of the good dews which descend from the heavens, and of the good fountains that spring up, and make the herbage of the earth to grow from beneath, and plenty of provision and wine.

OF THE DEW OF HEAVEN. The blessing is not that G-d give him of the dew of heaven for the dew descends in all places. Now had he said that G-d give him an abundance of dew, or that it come in its season, even as it says, Then I will give your rains in their season, that would have constituted a blessing. Instead, its meaning is as follows: Since above he mentioned G-d’s blessing, As the odor of a field which the Eternal hath blessed, meaning “which G-d had blessed for my son”, as referring back to the word b’ni (my son), thus making it “the field which G-d had blessed for my son.” Isaac thus continues his blessing by saying, “Just as He has blessed the field for you, my son, may He also give you another blessing, namely, of the dew of the heavens.” — that is, since G-d blessed him in the field by giving him success there in his hunt and by guarding him from death or any mishap — he now says, So G-d give thee, [as an additional blessing], of the dew of heaven, and of the fat places of the earth. It is thus a blessing of addition and abundance. It may be that the expression, And plenty (‘verov’) of corn and wine, is written in the Torah with an extra vav, [which should not affect the meaning], with the sense of the verse being: “So G-d give thee of the dew of heaven and of the places of the earth, i.e., plenty of corn and wine.”
In my opinion the correct interpretation is that G-d’s gift is steady and there is never any interruption in it. Therefore he says, “So G-d give thee for the extent of your days upon your land of the dew of heaven, and give thee of the fat places of the earth,” meaning the fattest of all lands, even as it is written, The beauty of all lands.
Now Rabbi Abraham ibn Ezra says that the prefix mem in the word mital, (of the dew) applies to itself and yet to another word, [namely, mishmanei (the fat places of the earth), which is then to be understood as] umimishmanei ha’aretz and from the fat places of the earth.
To Esau, on the other hand, he gave a blessing which mentions neither through a gift of G-d nor with abundance. Rather he said, “For you too I have reserved a blessing after him: of the fat places of the earth and of the dew of heaven shall your dwelling be.” That is “as long as you will dwell there,” thereby alluding that he will ultimately be destroyed and lost, for only as long as he will live will his lot be good.

ויתן לך, these words are a continuation of the words אשר ברכו ה' which precede them. They describe the type of blessing G’d will provide for Yaakov (Esau). מטל השמים, essentially what is known as the “blessing of the field.”

And abundance of grain. Sufficient that he would be able to support others.

ויתן לך...מטל השמים, something that you will require. Yitzchok did not mention rain, seeing that dew is always beneficial whereas rain can prove very destructive on occasion, as we know from the deluge or when it falls at the wrong time. These blessings were said not only to Yaakov (Esau) personally, but were meant to remain effective also for his offspring. Similarly, the blessing Yitzchok gave to Esau afterwards was also meant not only for himself but for his descendants afterwards.

ומשמני, the letter מ in the word מטל is to be understood as if it were written twice as if the Torah had written וממטל (the מ in front of the letter ש not having been a formative letter in the word ומשמני) Alternatively, the meaning of the word assuming the letter מ was a formative letter, it would be the same as the word משמני in the blessing Esau received in verse 39.

ויתן לך אלוקים, “and may G–d give you, etc.;” Yitzchok, -remarkably- addresses the attribute of Justice of G–d when we might have expected him to address the attribute of mercy. By doing so he implied that this blessing was conditional on the person on whom it had been bestowed being worthy of it. Nonetheless, when it comes to Esau, we see that Yitzchok does not include such a condition in his blessing, as in verse 39 in our chapter he reassures him by saying: “your dwelling will be in the fat parts of the earth and you will enjoy the dew from above. You will live by the sword, although you will serve your brother.” He adds that on the contrary, the blessing given to his brother Yaakov is conditional on his not making your life intolerable.” [my choice of words. Ed.] It is noteworthy that in his lengthy prayer after consecrating the Temple he has built, King Solomon attaches the condition of worthiness when speaking of how G–d shall respond to the prayers of the Israelites, whereas when speaking of gentiles coming to Jerusalem to pray there, he does not attach such a condition but asks G–d to grant the wishes of such gentiles, regardless. (Kings I 8,32-43) Solomon is concerned that the gentiles should not get the impressions that our G–d is someone with Whom one needs to bargain. Yitzchok was concerned about the same when realising that Esau, if he insisted on his blessing surely credited G–d in heaven with being the One Who would provide its becoming reality. If he did not think so, why would he demand it? [When G–d fulfills the requests of a gentile, without regard to his worthiness, if the gentile as a result does not become G–d fearing and grateful, he will find that the very fulfillment of his request will eventually boomerang and he will more than lose its benefit. Ed.] You will also find that there are ten different categories of blessings, corresponding to the ten utterances used by G–d when He created His universe, and corresponding to the Ten Commandments. These blessings were pronounced on seven different occasions. 1) here; 2).Genesis 28,3: ואל שדי יברך אותך and the attribute of G–d known as shadday will bless you. 3) Genesis 28,13 (to Yaakov); 4) Genesis 32,27: “I will not let you depart (Yaakov to the angel he had wrestled) until you bless me;” 5) Genesis 35,9: “G–d appeared to Yaakov;” 6) Genesis 46,3; and Genesis 12,2, where G–d blessed Avraham for the first time. Avraham was blessed by G–d on seven different occasions. The Jewish people were given seven different (days) that are holy, most of them festivals: Sabbath, Passover, Shavuot, Rosh Hashanah, Yom Kippur, Sukkot, and Sh’mini Atzeret. There are seven stars that serve the sun and the moon in their orbits. [The author adds some astronomical observation that have to do with the time to plant and harvest being related to sunset and sunrise which I have not understood. Ed.]