Barnes notes that the phrase "the twelve tribes" implies that the benediction which Jacob gives refers to the whole tribe, not just the heads of the tribe, which I thought was an interesting observation.
Gill's Exposition of the Bible follows suit, for he notes that there were actually 13 tribes (2 springing from Joseph: Ephraim and Manasseh), but Levi had no territory [apart from cities in each tribal area] in the land of Canaan, which was divided into 12 parts. He believes that this shows that Jacob's predictions are not just about the individual patriarchs of the tribes, but each tribe as a whole.
Gill also notes that although some of the tribes (think of Reuben, Simeon and Levi) may seem to be cursed rather than blessed, the majority of the tribes were clearly blessed, and what Jacob said about the others (by way of correction and rebuke), might become a blessing to them for their good. Also, they all had a part in the blessing of Abraham, Isaac and Jacob as it related to the land of Canaan.
Keil and Delitzsch's Bible Commentary on the OT also notes that although Reuben, Simeon and Levi were put down through their own fault, each received a share in the promised blessing.
Clarke's Commentary on the Bible notes that every one of Jacob's sons received their appropriate blessing (guided by the Spirit of prophecy). All of the most important events which would occur during the following generations were outlined, and the prominent characteristic of each tribe was revealed. Also, he notes that some comparatively obscure references to the coming Messiah and the redemption of the world by him were made in these blessings.