Blackett, who was MediaCom’s chief executive until December 2015, replaces Stephen Miron as the Nabs president. The Global group chief executive had been in the role for two-and-a-half years.
Blackett will be expected to play an instrumental role in driving Nabs’ new vision focusing on industry wellbeing.
At an event in December, Miron described its three pillars as supporting individuals, building a community and sharing expertise.
Last week, Nabs appointed Fold7 as its first retained ad agency. It will work on brand advertising and PR.
Diana Tickell, the chief executive of Nabs, said: "Nabs’ new strategy is launched at a time when we have seen continuing uplift in calls to our Advice Line and in particular a 68 per cent increase in calls relating to emotional support.
"In Karen, we not only have an outstanding figurehead, but also a hugely inspirational person."
Blackett was awarded an OBE in 2014.