Israel did not bless his grandsons in this way simply because they were "nice boys." Rather, he had come to understand the substance of the promises God had given him, his father Isaac, and his grandfather Abraham. Accordingly, he blessed the boys, as Hebrews 11:21 states, "by faith." His conviction that those promises were sure led him to bless his grandsons as he did.
It is important to understand the first part of the blessing. Israel granted none of his own sons the birthright blessing. That went to Joseph's sons instead. That is why he goes out of his way to inform Joseph, "Your two sons, Ephraim and Manasseh, who were born to you in the land of Egypt . . . are mine; as Reuben and Simeon, they shall be mine" (verse 5). As part of the blessing itself, he makes his wishes clear; the brothers are to bear the name of Israel: "Let my name be named upon them" (verse 16). This is important: Ephraim and Manasseh were born in Egypt (Genesis 41:50-52). Jacob wants to establish legally that they were not Egyptians, but were of the family of Abraham and therefore part of the structure of promises given by God to the patriarchs.
With that important legality out of the way, Jacob continues to bless his grandsons. He blesses Manasseh, the firstborn, with greatness; he blesses Ephraim, the younger boy, by saying that he would be still greater, not only a people but a multitude of nations (Genesis 48:16, 19).
Much to the consternation of Joseph, Jacob crosses his hands, placing his right hand on the head of the younger boy, Ephraim, and his left hand on the head of the older boy, Manasseh (verse 14). This was unusual, as the right hand, signifying the greater blessing, generally was placed on the head of the older son. Jacob refuses to realign his hands, telling the concerned Joseph that his actions were no mistake. He had "guid[ed] his hands knowingly" (verse 14) when he placed them on the boys' heads. Jacob knowingly bestows the greater blessing on the younger son, Ephraim, reserving a lesser blessing for Manasseh, the firstborn.
Searching for Israel (Part Two): Blessings in Faith