It was a doctrine in the ancient world that the firstborn son inherently had a special role, without regard to his particular merits, except in extreme cases, and accordingly it was understood that the the firstborn son was worthy of both a unique blessing and a double portion of the inheritance. See for example Deut. 21:15.
At the time that this story originated, blessings, curses and vows were special categories of expression that were believed have the power to determine reality. Once spoken, they could not be changed or repudiated. In particular, the blessings of the forefathers were regarded as prophetic for the sons. (For an extreme example of how far commitment to the spoken word could be taken see the story of Jephtah of the Gilad in Judges 11:30-40.)
Isaac gives Jacob two blessing, as befits a firstborn: the servitude of nations and his brother (27:28-29) and the transfer of the promise of the land and of descendants originally given to Abraham (28:3).
Isaac does what he thinks he has to do according to the traditional rules, despite his suspicions, but Rebekah intervenes in accordance with the contra-traditional prophecy that she was given (25:23). This is the way that the Biblical narrative, as sacred history, explains the historical fact that Jacob, and not the firstborn Esau, was chosen for the leading role in fathering the twelve tribes.
The word "blessing" in this passage has at least three different meanings, two
of which are not familiar to us.
Issac's first blessing of Jacob (27:28-29) is presented as a will, or
unconditional last testament. This is indicated by the usage "I do not know
the day of my death" (27:2), that Rebekah repeats in (27:7). It is clear to
all of the participants in the drama that the point of this first blessing is
to settle, once and for all, the question of succession, the right of the
firstborn. Since the story hints that there was a conflict between Issac and
Rebekah in addition to the conflict between Jacob and Esau, it might have been
Isaac's belief that by making his testament known he could end the family
strife, and this would indeed be a blessing in the sense that we understand.
Since the testament was witnessed (at least by Rebekah and Jacob, if not by
the servants) it was binding, despite the trickery.
Isaac's second blessing of Jacob (28:3) is a blessing given in parting and
is the type of blessing that we are comfortable with. Even though Jacob is
leaving the land promised to Abraham, Issac makes it clear that the promise
of the land and of descendants now belongs to Jacob. In context, this blessing
indicates to the reader that Isaac is reconciled with the outcome of the
events that have transpired.
Isaac's blessing to Esau (27:39-40) is a prophetic blessing similar to the
"blessings" that Jacob gives his sons in Genesis 49. This type of blessing is not easy for us to understand. In addition to the promise of release from his
brother's yoke and the "fat of the land and the dew of the heavens", this
blessing also includes "you shall live by the sword", which hardly sounds
good at all, but forewarned is forearmed, so this gift of prophetic insight
is also considered a blessing.