Phosphate, (Parkerizing), with Suncorite paint baked on became the standard MoD metal rustproof finish in late 1944. The first guns to leave the factory finished in this process were Mk.V Stens. Rifles continued to be blued or "blacked" as the OP says for the duration of the war. Factory fresh Canadianicon and US manufacture rifles were finished in Dulite blue. Most well used wartime rifles in Britishicon service went through FTR and had the phosphate and Suncorite finish applied post war.