The preferred time for counting
The preferred practice is not to count until nightfall-which is determined by the appearance of three medium-size stars. There is a view which holds that for this halach one may consider nightfall at 50 minutes after sunset.

Some Shuls daven Maariv before the appearance of three medium-size stars*. If a person is in a Shul which davens Maariv during twilight and would like to count later during the proper time, he should, nevertheless, count with them but with out a brocho, (if he forgets to count later he will fulfill his requirement)

He should have in mind the following condition: (he say this to himself) “If I should remember to count later at night, I don’t want to fulfill my requirement for Sefira Haomer with this counting” After nightfall, he may then count again with a brocho.
This is only adequate only if he has express intent no to fulfill his mitzvah with the counting of the Shul. However, if one counts with the Shul during twilight-without this express intent-when he counts again after nightfall, a brocho is not recited.
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Adopted from the class of Rabbi Edi Zamil/Halachos of Pesach, Rabbi Shimon D. Eider pg 309/310 volume 2