Bloom has been Ad Age’s executive editor since 2002. He arrived at the Crains pub after launching UK b2b publisher Haymarket’s PRWeek as news editor and he later served as editor-in-chief. Creamer, another former PRWeek alum, is leaving Ad Age after nearly three years. He’ll serve as executive editor. Bloom’s last day at Ad Age is Dec. 21, though Klaassen is now effectively executive editor. She has been with the magazine since 2005 and has been credited with developing the annual Digital Issue and the Ad Age Digital Conference.

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The move happened pretty quickly, sources tell paidContent. Bloom essentially let staffers know about his decision last night.

Ad Age is considered to be in a stronger position than its Nielsen rivals, Adweek, Mediaweek and Brandweek, and the soon-to-be-former Reed Business Information’s U.S. mags Multichannel News and Broadcasting & Cable. But the b2b magazine market remains particularly weak, a victim of the wider media and advertising industry travails as well as distinct problems of running an expensive print operation during a transition to digital. While Klaassen’s successor as digital editor has yet to be named, it’s not clear if the company will hire additional staffers to fill the gaps left by her move and Creamer’s.

Update: Bloom told me he does plan to help create some new blogs for BM, but his initial focus will be on building up the current four. He doesn’t view the past few day’s shakeup in the b2b mag trade as sign that the time has passed for the decades-old model. “While I’d certainly say the b2b world is changing, I don’t think you can write off all the brands, because the best of them—and I’d include Ad Age in that list—will keep evolving.” Bloom told paidContent.