Blanc, a team-mate of Zidane when a multicultural France won the 1998 World Cup, is under pressure after attending a football federation meeting in November where reports say the idea of quotas for Arab and African youth players was discussed.
"I think he started doing a great job and he has a real project," Zidane, who is of Algerian heritage, told L'Equipe on Saturday in excerpts from an interview due to be published on Sunday.
"We must leave things as they are, it would be crazy if he left because of this."
Some France '98 players, notably Senegal-born Patrick Vieira, have attacked Blanc but others, including Didier Deschamps, have backed the coach, who has led France to the top of their Euro 2012 qualifying group after their woeful World Cup last year under Raymond Domenech.
Blanc "is certainly not racist" and "has never thought like that", Zidane added, before querying the alleged federation discussion.
"The idea of... discriminating (against) kids with dual nationality is absurd to me," he said.
A Reuters correspondent said Blanc returned to his home in Bordeaux on Saturday after spending time at a spa in Italy. He is due to be interviewed in twin inquiries from the federation and the Sports Ministry.
The initial findings of the ministry investigation are expected on Monday.
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