United's second-string, who defeated Elche CF's first-team 2-1 in Algorfa on Saturday, struck twice inside the opening 45 minutes at the Rovers' Academy following Kyle Crossley's opener and Santiago Muñoz's eye-catching free-kick.
Blackburn halved the deficit after the hour mark through Harley O'Grady-Macken's close-range effort before Murphy - who joined the Magpies' youth set-up from hometown club Galway United earlier this month - scored in the dying stages, producing a diving header from Jay Turner-Cooke's in-swinging free-kick.
The 18-year-old left-sided centre-back, who has captained Republic of Ireland at under-18 level, told nufc.co.uk: "It's always nice to score - hopefully I can get one in a competitive match next time rather than a friendly.
"It was a great ball in by Jay and the wind took it towards me so I knew it was going to land on my head. I had to stretch to get there and managed to get a touch to turn it in.
"It's always good to win. At the moment, it's all about getting minutes in the legs and making sure we're fit for the season ahead but we're doing well and it's been a good start.
"It gives us a lot of confidence that we've been good in front of goal recently. We've worked a lot on finishing in training and have good players at the top of the pitch who are able to put the ball in the back of the net.
"We have plenty of goals all around the squad, too, not just up front."
Intending to play just the second-half, Murphy instead completed 68 minutes of the friendly encounter in Lancashire following Remi Savage's early withdrawal after an ankle issue.
The Annaghdown-born defender, however, revealed he relished the opportunity to gain further game time with his new club in preparation for the beginning of the 2022/23 campaign.
The promising teenager, who made his first-team debut for Galway at the age of 16, added: "It's good to get more minutes than I expected. Obviously I wouldn't say I was happy about that, with Remi getting injured, but it's always good to get as many minutes as you can. The more, the better at the moment.
"Once we got the first goal, we gained more of the ball and dominated, with Santiago scoring the second from a free-kick.
"I don't think we dominated as much in the second-half because the ball was going back and forth. However, we scored late on to draw the second-half if you like."
