3. When a person is asked by a colleague after sunset: "Which [day of the omer] is counted tonight?" he should reply: "Yesterday, we counted such and such." This is because if he would say to him: "tonight is such and such," [the person answering the question] would not be permitted to recite a blessing later when he actually counts [the omer] (1).
(1) The period between sunset and the appearance of three medium-size stars is called "bein hashmashos," during which there is a doubt as to whether it is halachically night or day. due to our inability to determine the precise moment that "night" begins. For Biblical mitzvos, we are stringent, but for rabbinical mitzvos we are lenient regarding this period. Therefore, when it comes to keeping Shabbos, we consider "bein hashmashos to be night, and thus from sunset onwards, we refrain from performing activities prohibited on Shabbos. However, according to the majority of authorities, the counting of the omer nowadays, is only a rabbinical obligation (because there is no Beit Hamikdash, and hence, no omer offering); therefore, we are lenient, and consider someone who counted during "bein hashmashos" to have fulfilled his obligation (that is, we consider the period to be "night" for the purposes of counting the omer).
It is for this reason that the person who answers the question after sunset by saying "tonight is such and such", has fulfilled his obligation of counting, and cannot say a blessing when he counts again after the appearance of three stars. However, it is not correct to place oneself into a situation of doubt, and therefore, one should count after the appearance of three stars, when it is definitely considered "night" (See Shulchan Aruch 489:2 and Mishna Berura there).
In the case discussed in today's halacha, if one just answered the person by saying the number of the day, without saying "TONIGHT IS..." OR "TODAY IS...", he has not fulfilled his obligation, and may count again later with a blessing. Also, if it is past the sixth day, when one has to mention the number of weeks as well as the number of days, one has not fulfilled one's obligation just by saying the number of the day, without the number of weeks (Mishna Berura 489:20-22).
Another issue involved here is whether or not one requires intent to fulfill one's obligation, at the time that one is performing a mitzva ("Mitzvos tzrichos kavana"). Although we rule that one must have intent to fulfill one's obligation, in order for one's performance of a mitzva to count, when it comes to our case, we do not permit the person who said "today is such and such," to count again with a blessing, even though he didn't have intent to fulfill his obligation when he answered his friend's question; this is because when it comes to whether or not to recite a blessing, we are stringent and take into account the opinion which states that one does not need intent to fulfill one's obligation when performing a mitzvah (ibid).