The announcement was made in a video posted on X, featuring Conceicao saying: "I am Sergio Conceicao, I came to make history with Al Ittihad."
Blanc’s exit came a day after Al Ittihad's 2-0 defeat to Al-Nassr in late September, which saw the defending champions slip to third in the league standings.
Assistant coach Hassan Khalifa took charge on an interim basis, supported by U21 coach Ivan Carrasco.
Conceicao was dismissed by AC Milan on May 29 after a disappointing Serie A campaign that ended with the club in eighth place, missing out on European competition for the first time since 2019.
He had joined Milan in December and led the team to the Italian Super Cup title in his second match in charge.