There is a Rambam in Hilchos Brachos that says that when Chazal were kovea to make Brachos on mitzvos they were kovea that only on mitzvos being adam l’Makom.
In learning a Gemara in Megila I had the following haarah: We know that when Kohanim duchan they make a Bracha, it would seem after pondering it, that this Mitzva is a Mitzvah being Adam l’chaveiro. They give us a Bracha, which they have special kochos to do, in order that we should prosper in the different areas they bless. Seemingly, that is a mitzva bein adam l’chaveiro. If so, according to the Rambam Chazal should not have been mesaken to make a bracha.
The Aruch Hashulchan at the end of Choshen Mishpat expounds on the question as to when brachos are and are not to be made. He himself suggests the following rule: Only mitzvos which are inherently clear that they are done as mitzvos, and that are done because of Kedushas Yisroel (he words it that way) were given a bracha. Mitzvos which could be viewed as things that are to be done bsechel, do not get a bracha. That is that although when a Yid does it, he does it because the Torahs said so, still a Bracha was not niskan.
I was thinking that possibly we can suggest that that is the deeper understanding of the Rambam. Since most bein adam lchaveiro mitvos are things that we would find (to some degree) by most nations, they are not clear to be mitzvos and therefor do not have a bracha. If that is the case then we can answer our question. True, Duchaning is bein adma lchaveiro however it is clear that it is a mitzva. Firstly, we do it in middle of davening or in the Mikdash during the Avodah. Secondly, only certain people, Kohanim, give the Bracha which again is indicative of that fact that it is not just a regular blessing. We can add that actually in the Bracah itself is says בקדושתו של אהרן, making it very clear that this is not a regular Bracha but one given by Kohanim because of their kedusha.
I look forward to your comments.
Shua
HI Shus, this is a great distinction and I agree this could help answer your question...however I think it may bring up some other issues, like for example brachos hoda, is that not something that is secheldick in nature? Grated the specific ways and wording perhaps not but in general?