C. F. Keil and Franz Delitzsch on Genesis 9:26

Gen 9:26

In contrast with the curse, the blessings upon Shem and Japhet are introduced with a fresh “and he said,” whilst Canaan’s servitude comes in like a refrain and is mentioned in connection with both his brethren: Blessed be Jehovah, the God of Shem, and let Canaan be servant to them.” Instead of wishing good to Shem, Noah praises the God of Shem, just as Moses in Deu 33:20, instead of blessing Gad, blesses Him “that enlargeth Gad,” and points out the nature of the good which he is to receive, by using the name Jehovah. This is done “propter excellentem benedictionem. Non enim loquitur de corporali benedictione, sed de benedictione futura per semen promissum. Eam tantam videt esse ut explicari verbis non possit, ideo se vertit ad gratiarum actionem” (Luther). Because Jehovah is the God of Shem, Shem will be the recipient and heir of all the blessings of salvation, which God as Jehovah bestows upon mankind. למו = להם neither stands for the singular לו (Ges. §103, 2), nor refers to Shem and Japhet. It serves to show that the announcement does not refer to the person relation of Canaan to Shem, but applies to their descendants.