The Parable of Two Merchants
Two merchants sold their wares at the same market. The first merchant sold the finest quality linen. He was always happy when his customers were knowledgeable, for such clients could appreciate the value of his merchandise and would not argue about the price.
The second merchant sold a cheaper, lower quality cloth. He was unhappy when discerning customers arrived, for they would quickly notice the defects and shoddy workmanship in his merchandise. His preferred ignorant clients who didn't know better. Or even better - customers with poor eyesight, or those arriving at dusk, when the defects in his cloths would be difficult to discern.
Balaam's Special Blessings
The Midrash teaches that "Everything that Israel enjoys in this world - is from the blessings of that evil man [Balaam]." But in the future, we will benefit from the blessings of the Avot - Abraham, Isaac and Jacob. The Midrash supports this idea that the blessings of the Avot are reserved for the future from the verse, "The Eternal your God will guard for you the covenant and the kindness that He promised to your fathers" (Deut. 7:12). God will save the kindness He promised to the Avot for the future.
Why are the blessings of the Avot only for future times? The Maggid explained this Midrash as follows:
The Avot were like the astute customers in the parable. They were great and holy men, and they could appreciate a generation of spiritual greatness and holiness. With a full heart, they blessed generations of future times, generations that will be filled with knowledge of God.
But who would bestow a generous blessing for mediocre generations, for the Jewish people during times of exile and strife and spiritual shallowness? To bless these generations, God chose a prophet who was nearly blind, one who would fail to notice their failings and foibles.
For this reason, Balaam stated that he was blind. Balaam needed to explain: Why was he blessing Israel? Why did God need or even want his blessings? But Balaam's blindness was his strength. He only saw what God wanted him to see.
Thus the Jewish people, even in their current imperfect state, benefit from Balaam's blessings.
(Adapted from Mishlei Yaakov, pp. 360-361)