Blackett, who will retain her current role as chairwoman of MediaCom UK for six months, has been appointed with immediate effect. She will oversee 17,000 staff in dozens of agencies spanning media, creative, design, branding, data and public relations across WPP’s £2 billion-a-year UK operation.
She said her priorities included getting WPP's agencies to work "in a more joined-up and collaborative way", developing new products and "managing and diversifying the talent base".
Blackett maintained that WPP, the dominant agency group in UK advertising with close to 40 per cent of the agency market, has "headroom" to grow further organically and she identified "high-growth" and e-commerce clients as sectors where "we don’t have a big enough market share".
She said: "If you look at the top 100 global brands, then we're doing pretty well. But if you look at another metric like 'high-growth' businesses, we could be doing a lot better."
Sir Martin Sorrell, the chief executive of WPP, described Blackett as "an exceptional and inspirational leader" and "a tireless champion of diversity in our business, in the wider sector and in public life in general".
He said in a memo to staff: "Karen will focus on our core strategic priority of horizontality: ensuring our companies work together effectively and present a seamlessly integrated and simplified offer to clients. She will also support efforts to attract and retain the best and most diverse talent, generate cross-group business opportunities and identify potential acquisitions and investments."
Blackett, whose parents grew up in Barbados before moving to Britain, was named the "most admired" business leader in the UK ad industry in a survey of agencies for Campaign’s 2017 School Reports.
She acknowledged that her promotion to WPP country manager has symbolic importance because she is black, female and "from a working-class background".
She said: "The ad industry is not diverse enough. It doesn’t reflect the consumer base that our clients are targeting."
Blackett hopes to apply some of the lessons that she learnt at MediaCom, Britain's biggest media agency, and its predecessor The Media Business, where she started in 1995, after spells at CIA and Zenith.
"When you get diversity of thought, you get better answers," she said. "It’s the route to creativity."
Blackett, who was UK chief executive of MediaCom from 2011 to 2015, sees parallels between her new role as WPP country manager and her past job as MediaCom's chief operating officer for EMEA when she worked with "a group of incredibly talented CEOs" from different countries to collaborate more effectively.
Her promotion was announced at the same time that Group M, WPP’s media-buying arm, named a new UK CEO.
This story originally appeared on PRWeek sister title Campaign