Question
If Bedieved one does not have Rashi Tefilin and only Rabenu Tam, is one allowed to say a bracha on Rabenu Tam?
Answer
ב"ה
Shalom
According to the Shulchan Aruch, one does not say a beracha on Rabenu Tam tefillin. ( Shulchan Aruch chaim 34:2).
If one does not have Rashi Tefillin and by the time he gets them the time of Tefilla will have passed, he should put on the Rabenu Tam tefillin without a Bracha. (Yalkut Yoseph)
All the best