Why must the omer be counted at night?
I have been taught that we count the omer at night with a b'racha. Counting at night makes sense; it's the first opportunity to fulfill the commandment each day and we should rush to fulfill a mitzvah.
I've also been taught that if you forget until the next morning, you do still count but you don't say the b'racha, either for that day or any following day. Once you miss a night you've broken the chain and are no longer fulfilling the commandment to count, but you should count anyway without the blessing.
My question is: why is counting by day (when it's still that day of the omer, after all) so wrong that you can't say a b'racha, then or at any time following? I can see why it would be preferable to count at night, but this doesn't seem like missing, say, one of the three daily prayers, and the one you missed has now "expired". It's the Nth day of the omer all day, isn't it?
What is the reasoning behind this halacha? Or have I learned it wrong?
(No, I haven't missed; I'm just curious.)