McCarthy, who netted at Anfield earlier in the season, bagged his tenth of the season early in the second half, while Brad Friedel was in inspired form against his former club throughout.
Mark Hughes made just the one change to the side that lost 6-2 to Arsenal on Saturday, with McCarthy returning to partner Shabani Nonda up front and Michael Gray the man to make way.
Liverpool made three changes to the side that defeated Watford 2-0 at the weekend, with Peter Crouch, Mark Gonzalez and Sami Hyypia coming in for Luis Garcia, Jermaine Pennant and Dirk Kuyt - who all dropped to the bench.
Craig Bellamy, who was making his first return to Ewood Park following his summer departure to Rafa Benitez's side, was immediately in the action. After taking the ball down the right, he centred for Crouch, whose towering header was comfortable for Friedel.
Five minutes later, Friedel again did well to deny John Arne Riise's dipping shot from distance, before doing even better to grab the loose ball.
On 17 minutes, the big American made another great stop. Gonzalez played the ball into the area, Jamie Carragher poked it forward and Bellamy turned fellow Welshman Robbie Savage to fire goalwards, but Friedel smothered the ball at point-blank range. Immediately up the other end, Nonda headed wide with a clear sight of goal.
Two minutes later, Morten Gamst Pedersen stung the gloves of Jose Reina from long range, before the Norwegian piled on the pressure with three consecutive corners from the left.
At the midway point of the half, Savage got up well with Steven Gerrard, the Liverpool skipper was only able to divert the ball into the path of McCarthy, who turned past Hyypia before firing a powerful left-foot strike into the arms of Reina.
A minute later, Bellamy showed good pace and balance to scamper away past Andy Todd on the left edge of the box, Friedel committed himself, but when Bellamy pulled the ball back, Brett Emerton was on hand to mop up the danger.
On 34 minutes, a dream move by Liverpool saw Xabi Alonso play a raking 40-yard pass out wide to Riise, the Norwegian headed the ball into the centre and Bellamy hit the outside of the post with a steered right-foot shot.
A minute later and Friedel made his best save of the half. Steve Finnan picked out the unmarked Crouch in the six-yard box, but with the goal gaping, the England striker's tame header was brilliantly kept out by Friedel.
But whilst the visitors had the better of the first half, Rovers came flying out of the traps after the break and took the lead on 49 minutes.
Man-of-the-match Tugay displayed some audacious skill on his approach to the Liverpool box, before feeding the perfect throughball for Gamst, the Norwegian centred for McCarthy, who arrived on cue to side foot the ball home.
Liverpool responded well and following a good period of possession on the hour, the ball broke to Bellamy in the box, but as the former Rovers striker smashed a left-foot strike off the bar, the whistle blew for an infringement.
As the game entered the final quarter of an hour, Friedel's heroics went from strength to strength. He first denied substitute Kuyt from close range, before keeping out Spanish duo Garcia and Alonso with sublime saves.
Five minutes of time added on was played, but Rovers held on for a much-needed Christmas bonus.