Substitute Roberts scored for the second time in as many games to condemn the Magpies to a third home defeat of the season following wins for Blackpool and Stoke on Tyneside, and their sixth in as many attempts by Rovers.
Andy Carroll looked to have snatched at least a point for the home side with a towering 47th-minute header to further enhance his England credentials, after Morten Gamst Pedersen had given Rovers a third-minute lead.
But on a night when midfielder Joey Barton once again found himself at the centre of controversy when he appeared to punch Pedersen in the stomach after the pair had brushed shoulders, it was Allardyce who had the last laugh. There is no love lost between the former Bolton manager and Newcastle following his eight-month reign on Tyneside.
He understandably believes he was given nowhere near enough time to make his methods work at St James', although his critics will point to the brand of football he asked his team to play.
Blackburn adopted a game-plan which involved closing down space in the middle of the park to deny a supply line to Carroll and in return, attempted to hit their hosts on the break at pace.
The ploy proved successful in the opening 45 minutes as the Magpies were unable to reproduce the kind of flowing football which characterised their thumping home victories over Aston Villa and Sunderland.
Rovers were helped in no small part by the gift of an opening goal within three minutes of kick-off when the normally reliable Cheik Tiote surrendered possession to Brett Emerton inside his own penalty area and could only look on in horror as Pedersen blasted a low right-foot shot across goalkeeper Tim Krul and into the bottom corner.
Newcastle's response was initially lukewarm, other than a deflected eighth-minute Jose Enrique shot which dropped just wide, as the visitors' muscular back three of Christopher Samba, Ryan Nelsen and Gael Givet coped with the aerial threat comfortably.
Indeed, with wide midfielder David Hoilett and overlapping wing-back Pascal Chimbonda repeatedly exploiting the space behind right-back Danny Simpson, Blackburn looked capable of adding to their tally.
Pedersen saw a 16th-minute strike clip Carroll and slip past the post and then thumped a free-kick into Krul's midriff on the stroke of half-time.
However, in the meantime, the Magpies had finally established a momentum, although without ever troubling Paul Robinson.
Samba blocked a Tiote piledriver at source and Kevin Nolan fired over, but it took the home side 42 minutes to get behind the Blackburn defence.
Jonas Gutierrez's crossfield pass put Barton in on the right, but the ball skipped up off the turf and his right-foot shot flew harmlessly behind.
Hughton opted for change at the break when Shola Ameobi failed to reappear and was replaced by Nile Ranger, and his side was back on terms within two minutes of the restart in a fashion Allardyce would have greatly appreciated had it come at the other end.
Barton's floated free-kick allowed Carroll to get up above Samba to power a bullet header into the top corner to further enhance his reputation.
In a frenetic passage of play in the immediate aftermath of the goal, Barton shot wide after creating space for himself and was then involved in a clash with Pedersen at the other end after Hoilett had been booked for diving.
The former Manchester City midfielder appeared to flick out at the Norwegian's midriff with his hand, although seemed astonished by his reaction.
Gutierrez failed to test Robinson as much as he might have with a 57th-minute strike, and with Roberts having replaced lone striker Nikola Kalinic, Rovers set about the task of trying to restore their lead.
It was a case of more brain than brawn as the two sides slugged it out in the search for a winner with the game approaching its final 20 minutes at breakneck pace.
But with Newcastle doing most of the pressing, they were caught by a sucker punch when Roberts muscled Mike Williamson off Robinson's clearance and tucked the ball past Krul eight minutes from time.
A final flurry failed to yield salvation for the Magpies, who once again succumbed to comparatively modest opposition on their own park.