Mayor Bloomberg just formally announced the long-speculated departure of NYC’s longest-serving schools chancellors, Joel Klein, and introduced his replacement, Hearst Magazines Chair Cathie Black.

With his selection of Black, Bloomberg has followed the same model that led him to tap Klein back in 2002. Prior to joining the administration, Klein was chairman and chief executive officer of Bertelsmann, Inc. one of the world’s largest media companies.

Klein has been a controversial and sometimes polarizing figure, but Bloomberg today said “no one deserves more credit” for turning a public education system that was a “case study for dysfunction and turned it into one that the Obama administration has hailed as a national model.”

“I think it’s fair to say he leaves a legacy of achievement that makes him one of the most important and transformational figures of our time…his legacy will go on,” Bloomberg said.

Klein had initially agreed to serve just two terms, but stayed on, Bloomberg said, to help the mayor select a successor.

UPDATE: Klein is returning to the media sphere, heading to News Corp. to become an executive vice president in the chairman’s office.

Maggie Haberman notes that Klein has long had a strong relationship with News Corp.’s Rupert Murdoch and the Post has long been supportive of Bloomberg, particularly when it came to his successful run for a third term. She also points out that News Corp. is, technically speaking, a competitor of Bloomberg News.

This is a big deal for Bloomberg, who has made turning around the city’s public school system a major focus of his mayoralty.

However, Klein’s departure could help improve Bloomberg’s relationship with certain members of the Senate Democratic conference, who are very anti-Klein and tried without success during the debate over mayoral control last summer to reduce the chancellor’s power.

Also, during the mayor’s race last year, former NYC Comptroller Bill Thompson, the Democratic nominee, said Klein should be fired.

Interesting: This news comes on the very day the Staten Island Advance published a story based on an editorial board meeting with Klein in which the outgoing chancellor took some shots at the UFT and also predicted the next round of budget cuts could bring teacher layoffs, more school bus cuts and bigger class sizes.

Welcome aboard, Chancellor Black!

464-10 (Cathie Black – Chancellor)