If I say, "Tonight will be the X night of the Omer", why is that considered as I've already counted? If I say, "G-d is blessed for creating the fruit of the tree", don't I still need to make the Bracha is hebrew before eating the apple?
See Shulchan Aruch Orach Chaim 489:4 that when one says tonight is such and such during Bein HaShemashos (the period of twilight when it is possibly day, possibly "night") one may no longer count again with a blessing. The M.B. seif katan 21 says that one has fulfilled ones obligation b'dieved (after the fact) even not having said "of the Omer". Since one has already effectively "counted" then and subsequent counting is not the fulfillment of a mitzvah and a blessing is unnecessary.
The Shulchan Aruch continues to explain that if one said tonight is such and such while it is still day, i.e. prior to Bein HaShemashos the time for counting has not arrived at all and one may count at night with a blessing.
Nevertheless it seems that many are careful even prior to nightfall to say "last night was such and such night of the Omer" leaving us to infer what tonight is. This is not halchically necessary but it seems to me that it has the very positive effect of reminding people to be careful about this halachah for when it is Bein haShemashos.
It is a Bracha Blessing on the performance of a Mitzvah, hence once you have stated the day you have already preformed the Mitzvah and no longer can make the Bracha because it is alredy done,we have a similar concept in picking up the Esrog we pick it up upside down because we do not want to fulfill or obligation untill after the Bracha hence we do it with a Shinu:An ALteration from the regular approach in order to prevent this very Problem.