The Torah tells us that Yaakov blessed his son Yosef, but a closer look at the blessing that follows indicates that the blessing was actually addressed not to Yosef, but to his sons, Ephraim and Menasheh (בראשית מ"ח:ט"ו). Why, then, does the Torah say that Yaakov blessed Yosef?
The Ramban (שם) writes that the Torah here is showing us that there is no greater blessing for a father than the wish that his children should take after him and become good people. Therefore, the blessing that Yaakov gives Ephraim and Menasheh is the greatest blessing Yosef, their father, could possibly have received.
The Ramban says further that Yosef at that time had no other children, and therefore every blessing that could be given to Yosef would manifest itself through these two sons. He then suggests that any other children that Yosef might later have would be blessed with the blessings of Ephraim and Menasheh, and he says that in fact Yosef did eventually have other children, who are not mentioned in the Torah, because they are included in the families of Ephraim and Menasheh.