Therefore, in your situation where one forgot to recite the blessing and count on a Friday and only remembered after accepting an early Shabbat, it would be permissible to count the missing day before it turns dark. This is based on the rulings and comparisons made between the counting of the Omer and the counting of Niddah, where the actual time for counting should be at nightfall.
It is important to remember that if one prayed with the congregation earlier in the day, they can count with them without a blessing, but if they remember at night, they should bless and count then. It is always best to consult with a rabbi or halachic authority for specific guidance in these matters.
Regenerate Accordingly, one may still count the Omer of the previous evening even after the congregation has prayed [an early] Ma’ariv.
Obviously, then, if the circumstance has so occurred, one may count after shekiah for that evening and continue every subsequent evening with a blessing.
In the merit of our fastidious performance of this mitzvah, may it be His will that we soon fulfill the complete mitzvah and bring our Omer to the third Beit HaMikdash, speedily in our days.