The Chidushim u’Berurim on Shas (37) maintains that the days a ger counted before his conversion do not combine with the days he counts afterward. He bases his decision on the Gemara (Yebamot 22a) that considers a convert similar to a child who has just been born. In effect, the convert is a new person. Thus, actions he performed before his conversion are separate from those he performs afterwards.
Although the Gemara we quoted above regarding peru u’revu seems to indicate that actions taken before conversion affect one’s mitzvah obligations after conversion, that Gemara might represent an exception. As R’ Yochanan states, the ger has fulfilled peru u’revu because his children, as a matter of empirical reality, are here on this earth. The mitzvah of peru u’revu was given to Adam so as to populate the world. Since the ger’s children are alive, he has fulfilled the miztvah. In general, though, a ger is considered a newborn child and the days of omer he counted before his conversion cannot combine with the days he counts afterward to constitute “sheva shabbatot temimot.”
In Responsa Chesed Le’Avraham (Vol. 2:56), we find a conclusion to the contrary. There we are told that we should only be concerned that “sheva shabbatot temimot” are lacking when one did not count on those days on which one was obligated to do so. However, in the case of a ger, since his obligation only starts after he converts, there is no issue.
Nevertheless, we follow the majority opinion (Birkei Yosef 120; Responsa Pri Haaretz Vol. 3:11; Shalmei Tzibbur p. 298; Aruch Hashulchan, Orach Chayyim 489:15). A ger who converts in the middle of sefirah starts counting afterward without a blessing.
Tractate Berachot (15a) states that saying a berachah before fulfilling a mitzvah is not crucial to the mitzvah’s performance. In other words, one has fulfilled the mitzvah even without saying a berachah. Thus, a person who counts sefirat ha’omer without a berachah has still fulfilled the mitzvah. Of course, if someone is in shul for Ma’ariv, he should listen to the chazan or rav recite the blessing. By answering “amen,” he will also fulfill the rabbinical requirement of saying a blessing by the principle of “shome’a ke’oneh.”
All of the above opinions and considerations apply to an individual’s personal mitzvah to count the omer. However, the rules are different if one is serving as chazzan. Since there might be individuals in the congregation who are relying on the chazan to fulfill their obligation, it would not be proper for him to recite the blessing on their behalf.
As far as an ordinary Jew is concerned, unless there is definite reason to suspect that he has not counted the omer until now, he has a chezkat kashrut – a presumption of integrity regarding his mitzvah observance – and may count on behalf of the congregation with a blessing. (See Kiddushin 49b and Rambam, Hilchot Ishut 8:5.)
May it be His will that in the merit of our counting the sefirah, our dreams of redemption which we anticipate as we approach the festival of Shavuot be fulfilled speedily in our days.