venivrechu vecha - and through you, will be blessed
The plain meaning of these words is that in proportion to all the curses that have been heaped upon you at this time, in the future all these nations will be amongst those who bless you.
A Midrashic interpretation (Bereishit Rabbah 39:19) of these words, "the dew and the rain [nature's way of spreading blessing] will be due to your merit." The words venivrechu vecha, and through you, will be blessed all the families of the earth, "were first fulfilled when Yaakov blessed Pharaoh (and the famine ended). They were fulfilled when Yosef revealed the meaning of his dream to Pharaoh, enabling him to take measures to insulate his people against the devastating effect of seven years of famine. Similarly, Dani'el interpreted Nevuchadnetzar's dream with beneficial effects for him. Whenever the Gentile nations experience troubles they will turn to the Jewish people for advice and blessings and we will reveal to them what to do.
A Kabbalistic approach sees in the words venivrechu vecha a message that the blessing will filter down from above, seeing that Avraham represents the highest emanation - so that he will be the first one to receive it from G-d - and he will channel it through the lower emanations. All the blessings the Gentile nations receive will come to them only via Avraham. This is also why David said, "praise Him, all you peoples,... for great is His steadfast love for us and His faithfulness endures forever" (Tehillim 117:1-2) The structure of these two verses indicates that the Jewish people and their relationship to G-d are the only reason that G-d suffers the existence of the other nations at all.