From this passuk we learn that when Yovel was practiced, the Bais Din Hagadol would instruct that forty nine years be counted, orally, until the Yovel year, similar to the mitzvah of counting the Omer, which obligates a person to count forty nine days until Shavuos (Rambam Shemittah Veyovel 10 1)
The Chizkuni wrote that even though the counting of the years to the Yovel is a mitzvas aseh, the sages of the Bais Din would not make a brachah before fulfilling it, the way we do before counting the Omer. This is because Chazal only instituted brachos for commandments that apply to everyone, and a mitzvah that was only imposed on the Bais Din Hagadol, such as this one, did not have a brachah instituted.
But the Ra’avad differs and holds that we do make a brachah on this mitzvah. He wrote: “And we recite a brachah on this counting at the beginning of each year, the way we make a brachah on the counting of the Omer at the beginning of each night. At the first shemittah they would say: ‘Baruch Atah Hashem... bemitzvosav vetzivanu al sefiras shnei hashmittah,’ or ‘Al sefiras shnei hayovel.’”
Sefer Aruch Hashulchan Ha’asid says that until the seventh year they recited “Al sefiras hashmittah,” and from the seventh year on they recited, “al sefiras hashmittah vehayovel.”
Sefer Hamitzvos LeRambam Aseh 140; Biur HaRa’avad LeToras Kohanim Behar 22; Chizkuni; Aruch Hashulchan Ha’asid Zeraim 30 7
