An early try from Alex Cuthbert put Blues on the front foot before a score from Rhys Priestland just before the break fired a Scarlets fight-back.
A tense and nervous second half saw both teams reduced to 14 men – as Robin Copeland and Liam Williams were both sent off.
A try from Ken Owens set up a grandstand finale at the home of Welsh rugby. But impressive Blues held firm to claim the vital win and move up to seventh spot in the league standings.
John said: “I was pleased with the way we played in the first half, but we only came in 14-8 up.
“The second half was nail-biting and we had so many chances to go further away, but Scarlets were always in there and that last few minutes, when we had the ball and kicked to touch, was a little nerve-wracking. We dug deep and deserved the victory.
“We will look at the red card to Robin, but I don’t want to comment on that now,” he added.
“From a rugby point of view, we are very pleased with the commitment and attitude of the players.
“With Gethin Jenkins, Bradley Davies and Matthew Rees back the demand for quality in training has gone up, it’s much sharper. You play as you train and that commitment was there from the off so we had the confidence to hold onto the victory.”
Blues now face Connacht and Scarlets in the closing weeks of the season.
John concluded: “That is three wins on the bounce now. It’s the first time we have beaten Scarlets in a while and we can take real heart from that.
“Six weeks ago people asked us what was left in the season and what are you playing for? But the attitude to get a positive end to the season has shone through and you can’t fault the boys.
“The final passage of play summed that up today. Dale (McIntosh) and the boys have put lots into the defence and we showed heart. We got off the floor and tackled again. The attitude was spot on and we put ourselves in the position to give everything for the cause.
“It was open in the last 10 to 15 minutes, it’s a big pitch and it seemed to go for 10 minutes longer still. But we showed heart and we can build on that.
“We haven’t shown the table to the boys, haven’t shown then what is possible. We just play each game and see where we end up. Other people can look at that, we will just look at the next game. Performances will then look after themselves.”
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