blessing - Why don't we count the omer with a bracha during the day?

Seem that the commandment is to count, why do we make a distinction between counting during the day and at night?

Answer

The Torah commands us to count the Omer "מהחל חרמש בקמה" (Devarim 16:9), ("When the sickle 'begins' with the stalks"). Thus, it is assumed that the Omer should be counted when the stalks for the Korban Omer should be cropped.

The mitzva of קצירת העומר (cropping of the omer) applies at night (Menachos 71a), and it is disputed amongst the Rishonim whether it is acceptable if done by day.
Thus, the Shulchan Aruch (OC 489:7-8) rules (following the Rosh) that if one forgot at night, he should count by day, but without a b'racha (see Tur OC, 489).