1. Question: Why are there some poskim who hold that if one isn’t sure whether he fulfilled the mitzvah of Omer or not he should still say a blessing and count the Omer for that night but they would also hold that a tumtum should not say a blessing upon putting on tzitzis?
A. A tumtum is a person born without clear signs of genitalia, it might be the person has to much fat or skin in the area to make a judgment whether it is female or male therefore there is a halachic doubt whether it is obligated in the mitzvah of tzitzis, so he should wear the tzitzis in doubt but should not say a blessing because, safek brachos lihakel, we are lenient to not say a blessing when in doubt lest we might say Hashem’s name in vain.
B. Shouldn’t this opinion, also be concerned that the person might have said the Omer already and saying a blessing potentially again might constitute saying Hashem’s name in vain so when in doubt don’t make the blessing?
Answer: This opinion holds that when there is a doubt of whether there is a mitzvah to begin with then one should not say a blessing but by this case of the Omer there was a definite obligation the only question is whether it was fulfilled yet or not therefore he holds a blessing should still be said since a definite obligation exists.(See Dirshu Mishna Berura footnote 41 on Be’ur Halacha 489:1 “moneh viholech”)