'Lark Blue' was a British operation by the Special Operations Executive to re-establish contact with and continue organising groups in the Trondheim area of German-occupied Norway as well as to distribute arms dumps and prepare to receive 'Source' (14 March/October 1943).

O. Sørli and E. Gjems-Onstad were landed from the sea on 14 March with two radio sets, one other set having been landed a few weeks earlier. The 'Lark' organisation was still intact, and eventually two transmission sites were established in Trondheim, allowing 'Lark Blue' to come on air. Gjems-Onstad trained a local operator to replace him in Trondheim and attempted to set up a station in Kristiansund as 'Lark Brown'. In October 1943 both men left for Stockholm.