It could be that the notion of a berocho is mideoraiso, but the exact technical words to be used are miderabonon.
This is like Shabbos which is itself a deoraiso, but when it comes to the exact way of keeping Shabbos, there are midoraiso and miderabonon. So when you do not move a muktze object on Shabbos, this is miderabonon, although Shabbos itself is midoraiso.
Moshe
I think you're referring to the Ramban in Emor 23:24 who says that there is a Biblical edict not to "do hard work" on Shabbos, even if it does not involve the 39 prohibited acts, and the Rabanan added specific acts that one should not trouble one's self to do, since they lead to doing hard work.
In fact, we referred to a Berachah such as you describe here in the Insights to Berachos 40:3. In our case, though, your answer would not seem to apply. If it would be as you say, one would have to make some sort of "Berachah mid'Oraisa" every time he is uncertain as to whether or not the Berachah has been made, since "Safek d'Oraisa l'Chumra." The Halachah, though, only requires us to recite Bikas ha'Mazon under such circumstances, but not other Berachos.
Thanks for your comments,
-Mordecai