The incident happened at around 10.15pm on Sunday, March 12 when the men wearing dark clothing with their faces covered entered the Lock Mill Service Station on Bolton Road.
They threatened the cashier behind the till and made demands for money, but he ran into the toilets and alerted the police while the pair went behind the till and stole its contents.
They managed to get away with a small amount of cash and cigarettes, heading in the direction of Hollin Bridge Street.
DC Michelle Moffat from Blackburn CID said: “Thankfully they didn’t get away with much and the cashier wasn’t hurt, but he is understandably shaken up after being threatened when he was simply just there to do his job.
“If you saw these men in the area before or after the incident or you have any information that could help us, please get in touch by calling 101 quoting incident reference 1495 of March 12.”
Alternatively anyone with information can contact Crimestoppers on 0800 555111 or online at crimestoppers-uk.org.