The physical portion of the birthright is set in tradition and law. It’s very contractual in its way. But it’s only half of what Jacob needs to ensure his father’s legacy and to safeguard his mother.
Unfortunately, the other half is up to Isaac to bestow however he wishes. The blessing is a companion piece to the physical birthright. It generally follows the birthright, but it’s really up to the giver of the blessing (usually the family patriarch) to give it to whomever he wished.
Esau is still Isaac’s favorite son, so, while blind and near death, Isaac chooses to give it to Esau. And this is where Rebekah actually enters into the equation. She also wants Jacob to inherit, so concocts a way for this to happen, having Jacob dress up as Esau, complete with animal skins to fake his brother’s hairiness. It’s clearly a deception, and no amount of rationalizing will change the fact that Rebekah and Jacob conspired to fool Isaac. The only thing that can be said is that it was for the greater good.
Here is the text of Isaac’s blessing, delivered to Jacob, in full:
“Ah, the smell of my son is like the smell of a field that the Lord has blessed.
May God give you of the dew of heaven, and of the fatness of the earth, and plenty of grain and wine.
Let peoples serve you, and nations bow down to you.
Be lord over your brothers, and make your mother’s sons bow down to you.
Cursed be everyone who curses you, and blessed be everyone who blesses you!”
Isaac talks about the fields and yields of grain and grape. This is not a blessing for Esau. In many ways it sounds like a standard blessing of sorts since it mentions a whole thing about multiple siblings when it’s just Jacob and Esau.
This is a blessing for the fields and flocks to grow in strength and the overall influence of the family to increase as the blessings asks, “Let peoples serve you, and nations bow down to you” according to what God promised Abraham. Nowhere is mentioned anything about hunting or game. This is a farmer’s blessing, not a hunter’s. Esau doesn’t have the skill and experience to safely govern his father’s lands. Even with birthright and blessing, he would likely lose it all.
Like the birthright, the blessing is better in the hands of the son who can be responsible with it and protect the legacy. Sure, it’s underhanded to deceive Isaac, but Isaac also deceived himself by making Esau his favorite when Jacob was clearly the best son to take over.