As you assume, if one makes a declaration that nominally fulfills the mitzva of sefirat ha’omer – even without making a beracha and without the intention to thereby serve HaShem – he subsequently may not recite sefirat ha’omer with a beracha.[1] This is one of the cases in which we prefer to not have fulfilled a mitzva, so that we can still perform it properly by reciting a beracha.
However, although the statement, “Tonight is Lag BaOmer” (henceforth, “the statement”) does include basic elements ordinarily needed to fulfill the mitzva of sefirat ha’omer, it is possible that in this case one has not fulfilled the mitzva for a number of reasons.
First, there is an unresolved machloket regarding whether counting sefirat ha’omer by stating the day’s numerical gematria (lag = lamed gimmel = 33),[2] which is a secondary but accepted way of expressing numbers, is valid for sefirat ha’omer.[3] It is therefore questionable whether the statement fulfills the mitzva.
Second, the statement does not include mention of the weeks of sefira. The Acharonim debate whether one who has mentioned only the days and not the weeks has fulfilled his mitzva (starting with day seven). The matter relates to Ameimar’s opinion[4] that there is no need to count weeks at a time that there is no Beit HaMikdash in which to offer the korban ha’omer.[5] The Mishna Berura[6] concludes that one who mentions only the days should count again properly, but this case lacks the level of certainty to justify a new beracha.[7] (According to the Eliya Rabba,[8] the problem of leaving out weeks, which requires one to count again, applies only on days when the number of weeks changes – e.g., 28, 35).
The strongest reason to discount the statement as a possible fulfillment of the mitzva is that when one says it, he almost certainly does so without having in mind to fulfill the mitzva of sefirat ha’omer. The Shulchan Aruch[9] rules that one does not fulfill a mitzva in the absence of intent to do so, and the statement therefore should not prevent one from counting afterwards with a beracha. However, another halacha in the Shulchan Aruch[10] seems to contradict this: If one is asked before counting what day of the omer it is, he should answer what day yesterday was, because stating the current day compromises his ability to count later with a beracha. The Taz[11] says that the Shulchan Aruch must mean that avoiding saying the day’s count is just a stringency; if one were to say the current day, due to the lack of intention, he would b’di’eved count with a beracha later. However, many point out that the Taz’s claim does not fit the Shulchan Aruch’s language. The Magen Avraham[12] says that one should not make a beracha if he mentions the day’s count without intention to fulfill the mitzva, due to the opinion that intention is not critical for mitzva performance and the view that sefirat ha’omer is fulfilled without intention because it is only a Rabbinic obligation.[13]
While each individual reason to allow counting with a beracha after the statement is debatable, the combination of these reasons makes that prospect convincing in two possible ways. First, in other contexts, poskim[14] maintain that when there are specific indications that one intends to not fulfill a mitzva, he indeed does not fulfill it. In the standard case, when “Lag BaOmer” is used as the name of a semi-holiday, as opposed to as the gematria of the count, the statement would be precluded from fulfillment of the mitzva and a beracha could be made later.[15] The Mishna Berura[16] says that we would accept the aforementioned logic of the Taz in cases in which the week should have been mentioned and was not. Second, the coinciding of factors may create enough doubts against the chance that the mitzva was fulfilled to justify a beracha. Indeed, according to many opinions, we find cases in which we make a beracha on sefirat ha’omer when s’feik s’feika[17] indicates its appropriateness.[18] (However, that halachic phenomenon likely does not apply to every set of doubts.[19])
In short, it is unlikely that one has fulfilled sefirat ha’omer by noting that the day is Lag BaOmer. Therefore, one who has done so may still subsequently count with a beracha. However, it is worthwhile to avoid such a statement before counting and, when easily feasible, to rely on someone else’s beracha if he did.