Ian Burrell: The secret weapons of the most powerful woman in media

Karen Blackett is star-spotting over afternoon tea in London's Wolseley restaurant. She picks out Dennis Wise, the diminutive former England midfielder and David Dein, one-time Arsenal vice-chairman. But maybe the stars should be spotting Blackett, who has £1bn a year to spend.

Strictly speaking it's not her cash, but she runs a company that will pour that money into the British media economy and, in so doing, dictate the fortunes of the television, newspaper, radio, outdoor and online sectors. As the CEO of MediaCom, Britain's largest media agency for the past decade, Blackett, 40, is one of the sector's power players. Her client list includes Boots, Audi, NatWest, DFS and Direct Line.

Asked to identify key media trends, the first word off her tongue is Zeebox. She's referring to the social networking site for mobile devices that encourages "dual screening" by offering the latest Twitter or Facebook commentary on television shows you watch. Launched by former BBC iPlayer chief technology officer Anthony Rose last October, Zeebox is already living up to its boast that it is the "best thing to happen to TV since TV".

Blackett's love of Zeebox dual screening is good news for television advertising, but then MediaCom's client list also includes Thinkbox (the TV advertising body) and BSkyB, which bought a stake in Rose's company at the start of the year.

She plays down recent upheaval at Sky, saying it's business as usual, in spite of James Murdoch's resignation as chairman. Tom Mockridge, the News International boss, was a recent dining partner and she is impressed by the way he has done away with the executive floor at Wapping.

MediaCom is part of Sir Martin Sorrell's WPP empire, the biggest advertising group in the world. She endorses Sir Martin's view that the Olympics, the Diamond Jubilee, the football Euros and the US elections "should make this year buoyant for advertising revenue". But persuading businesses to spend their cash in this economic climate is not easy.

Blackett tells customers to reach out to mothers – "the CEO of the household" – because their brands are competing for a place on the shopping list. While most clients are employing "quite male-dominated workforces" to sell basic household services, MediaCom's top posts are nearly all filled by women, including chairman Jane Ratcliffe, managing director Claudine Collins and chief strategy officer Sue Unerman.

Blackett oversees a business science division of 45 econometricians who provide "data technology" to help clients understand purchasing behaviour. They helped drive a crowd of 5,500 to Alton Towers on a rainy day by offering cut price rides via groups on Facebook.

But her secret weapon is a "Real World Street" of 25 households of guinea-pigs that MediaCom has recruited and equipped with video-cameras in Blackett's home town of Reading, which is apparently typical of the consumer habits of the UK. The material they provide is shared with MediaCom's clients. The research shows that mothers who first saw the recession as a challenge to their budgeting skills no longer want to trawl for bargains. "The brands that help them find deals ... are the ones that will survive," says Blackett.

Increasingly, MediaCom is helping clients to fund their own media content. Home of the Future, a Channel 4 show in February, was co-funded by MediaCom client E.ON, which advertised around it and built a related campaign in social media. To explore similar projects, Blackett has established a 40-strong MediaCom Beyond Advertising division run by former BBC factual and arts commissioning editor Nick Cohen. It helped RBS fund a rugby documentary for Scottish Television. Other shows will be aimed at mothers, says Blackett. "Branded content is going to be a really important avenue for us to reach the CEO of the household."