How can you get the no name key blade in Kingdom Hearts Birth by Sleep?

The Oblivion Keyblade is not a synthesizable weapon and therefore cannot be created.

In Kingdom Hearts, the Oblivion can be found in the castle where you fight Riku in Hollow Bastion. Simply follow the interior of the area near the giant door. It is located within a chest.

In Kingdom Hearts II, the Oblivion is given to you in The World That Never Was by the character Riku in The Hall of Empty Melodies (in the Castle).

In Kingdom Hearts Coded/Re:Coded, the Oblivion is given to you upon completion of Hollow Bastion (pt.1).