veheyeh beracha - will be a blessing
The Sod of these words veheyeh beracha is that G-d had said: "up until now when I created My universe and it needed My blessing in order to endure I blessed Adam and Chavah as we read (1:28) 'G-d blessed them.'" This was repeated when Noach and his family required G-d's blessing after the deluge in order to rebuild mankind. At that point (9:1) the Torah wrote: "G-d blessed Noach and his sons," etc. From here on is the power to bless was entrusted to Avraham who could use it to bless whomever he saw fit to qualify for a blessing.