Blackmon was listed as probable for Sunday's NFL International Series game at Wembley Stadium because of a hamstring/groin issue that had also kept him out of practice on Wednesday.
Coach Gus Bradley says Friday's decision was ''precautionary.''
Blackmon has 25 receptions for 394 yards receiving with one touchdown in three games this season. Blackmon is perhaps the 49ers biggest worry on defense, with safety Donte Whitner believing his opponent's offense ''goes through Blackmon now. He's their No. 1 target.''
Bradley added that fellow wide receiver Stephen Williams was questionable to play.
AP NFL website: www.pro32.ap.org
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