Exciting!! A friend of mine from America bought an apartment in Israel and asked me to affix the mezuzas. Do I make a bracha?

Avade!! [Yiddish for "Why certainly"]

Every mitzva done by an agent is done with a bracha.

One exception. Ner Chanukah! If a shaliach lights for a person in that persons home and the homeowner is not present no bracha is made. This is unlike bedikas chametz, seperating Terumah, affixing a mezuza etc. etc. where a bracha IS made by a shaliach.

Why is Ner Chanukah different???

Rav Shlomo Zalman Auerbach ztz"l explains as follows: A shaliach is considered as if HE has performed the mitzva after the person obligated has passed on his power and rights to the mitzva as we know that there is a law called "arvus" that means if my friend is obligated in a mitzva it is as if I am obligated. Hence, the shaliach should make a bracha.

But Ner Chanukah is a mitzva on the HOME and only the homeowner is obligated to fulfill this mitzva. He cannot pass this obligation onto his friend. So when the shaliach performs the mitzva it is akin to a person who puts tefillin on his friends body. The shaliach is clearly not performing a mitzva and thus no bracha is required.