Rav Moshe Feinstein explains that Avraham merited greater distinction because he was the first to establish faith in Hashem (G-d). Although those after him reached greater heights, Avraham's accomplishments were more worthy. Maybe this can explain how Avraham himself became the blessing: Taking initiative and starting something you believe is important for society is a blessing on its own, because it lays the framework for others to build on it! G-d promised Avraham, and in turn promised us, that, if we become leaders and initiators, our initial efforts will never be forgotten and we will always be remembered as a blessing!