Tottenham Hotspur centre-back Younes Kaboul, who had been included in the preliminary list of overseas-based players, was ruled out with a serious knee injury.
Kaboul's knee injury flared up during Tottenham's final Premier League game against Fulham on Sunday. "He's not fit to play in the Euro," said Blanc.
Kaboul's injury led Blanc to call up the 23-year-old Mapou Yanga-Mbiwa, who has been marshalling the Montpellier defence all season.
Olympique Lyon midfielder Gourcuff has had a mediocre season and has not featured in a France game since a 0-0 friendly draw against Croatia in March 2011 but he is, according to Blanc, the only proper playmaker.
Blanc also named Loic Remy in the 26-man squad although the Olympique Marseille striker was ruled out of action for up to four weeks with a thigh injury on Monday.
"His chances of taking part in the competition are very slim," said Blanc.
The main debate was over Gourcuff, who has failed to meet expectations since he joined Lyon from Girondins Bordeaux two years ago.
"There will be another decision before we unveil the final list. We will assess the mental state and the place in the squad," Blanc said when asked about Gourcuff.
"His club performances are indefensible, but his statistics at international level are not that bad. And no-one else has stepped up [in the playmaking position]."
Gourcuff has scored two goals in his last six appearances with Les Bleus, showing qualities barely seen in his Ligue 1 games.
Blanc has until May 29 to name his final, 23-man squad.
France are drawn in Group D with England, co-hosts Ukraine and Sweden for the June 8-July 1 championship.
France preliminary squad:
Goalkeepers: Hugo Lloris (Olympique Lyon), Steve Mandanda (Olympique Marseille), Cedric Carrasso (Girondins Bordeaux).
Defenders: Gael Clichy (Manchester City), Patrice Evra (Manchester United), Laurent Koscielny (Arsenal), Philippe Mexes (AC Milan), Adil Rami (Valencia), Mathieu Debuchy (Lille), Anthony Reveillere (Olympique Lyon), Mapou Yanga Mbiwa (Montpellier).
Midfielders: Yohan Cabaye (Newcastle United), Florent Malouda (Chelsea), Samir Nasri (Manchester City), Alou Diarra (Olympique Marseille), Yann M'Vila (Stade Rennes), Yoann Gourcuff (Olympique Lyon), Marvin Martin (Sochaux), Blaise Matuidi (Paris Saint-Germain).
Forwards: Hatem Ben Arfa (Newcastle United), Karim Benzema (Real Madrid), Franck Ribery (Bayern Munich), Olivier Giroud (Montpellier), Jeremy Menez (Paris Saint-Germain), Loic Remy (Olympique Marseille), Mathieu Valbuena (Olympique Marseille).
