Blackwell will head the agency's accounts with Unison, Unite and accountancy firm Mazars, reporting to MD Gill Morris. He joins as an account director.
Connect has also hired John Slinger, a former aide to Anne Clwyd MP, as an account executive.
Both are new posts, but Blackwell takes on some of director Simon Redfern's responsibilities. Redfern joins Fishburn Hedges this month (PRWeek, 4 Aug).
Blackwell was at Camden from 2002 to May 2006.