The Rambam writes (Hil Chanukah 3:4-5)
כל שחייב בקריאת המגילה, חייב בהדלקת נר חנוכה; והמדליק אותה בלילה הראשון, מברך שלוש ברכות
The Rambam then goes on to say there is a mitzvah of saying hallel with a bracha, and continues:
אף על פי שקריאת ההלל מצוה מדברי סופרים, מברך עליה אשר קידשנו במצוותיו וציוונו, כדרך שמברך על המגילה ועל העירוב--שכל ודאי של דבריהם, מברכין עליו.The Rambam tells us not to be troubled by the fact that we say a bracha on the derabbanan of hallel because it is no different than megillah and eiruv where we also recite brachos.
Why does the Rambam address himself to this issue of how we are allowed to recite a bracha on a derabbanan only in halacha 5 when he speaks about hallel and not in halacha 4 where he speaks about the brachos on ner Chanukah? Those brachos are also on a takanah derabbanan, and furthermore, the gemara addresses itself to the question in the context of new Chanukah, not hallel!
Secondly, why does the Rambam prove that you can say a bracha on a derabbanan from the examples of megillah and eiruv and not from the case of ner Chanukah which he just spoke about in halacha 4?!
Based on the chiddush we quoted yesterday from R’ Ya’akov Emden, the Rambam makes fits beautifully. R’ Y.E. suggested that the brachos on ner Chanukah are different than all other brachos is that they serve to define the ner as a cheftza shel mitzvah and not just as a lamp in your living room. The brachos are part and parcel of the mitzvah of hadlakah, not a separate din.
Therefore, the Rambam was not bothered by why we recite brachos on ner chanukah. Without a bracha, it’s just a lamp in your living room – those brachos are a crucial component of the mitzvah. The Rambam was only bothered by why we can say brachos on other mitzvos derabbanan where the bracha is just an added element.
This also explains why the Rambam used the examples of megillah and eiruv and not ner chanukah to make his case for reciting brachos on all dinim derabbanan.
(I found this diyuk in Rambam in R’ Moshe Brown’s (no relation) sefer Ma’adanei Moshe, but he explains it just slightly differently [he says that there is a kiyum of pirsumei nisa inherent in the hadlakah which must be accomplished by the recitation of brachos], with a nafka minah l’dina, so ayen sham if you are interested.
G.U. in a comment yesterday also suggested that perhaps the ikar takanah is the hoda’ah through brachos and al hanisim and the hadlakah is just a means to provide a context to that expression. This is a beautiful sevara that explains a lot of other things as well...)