But King's New Zealand compatriot Casey Laulala helped the Blues take control as the game wore on, with second half tries from Taufa'au Filise, Jamie Roberts and Richie Rees stretching the Cardiff men into a clear lead.
Having led at the break, the Scarlets leaked 24 unanswered points in the second period and will now have to hope the Blues win the Amlin Challenge Cup in order to gain a back-door entry into the Heineken Cup.
The Scarlets went into the match boosted by out-half Jones extending his contract with the region until the end of the 2011/12 season.
But they were without another Lion in Matthew Rees, who was sidelined with a neck injury. The Blues were missing Andy Powell, Sam Norton-Knight, Tom James, Gareth Cooper, Tom Shanklin and Gary Powell.
In a fast-paced and well-contested first half, the Scarlets took the early initiative and Jones kicked 11 points, allied to King's early try, to give the hosts a 16-15 lead at half-time.
First blood was drawn by the Scarlets when centre King weaved his way over after appearing on the shoulder of Jones for the opening try.
But the home side's 7-0 advantage was wiped out in the space of five minutes, first by a Ben Blair penalty and then by a try for Jamie Roberts who collected a neat pass out of the tackle from Laulala and crossed in the left corner, with Blair's successful conversion making it 10-7.
However, a penalty goal and then a drop goal from the influential Jones gave the Scarlets a 13-10 advantage.
The Blues hit back with a try from lock Deiniol Jones who stretched for the line after another dash down the left by Roberts.
But from the restart, Cardiff hooker Gareth Williams was penalised for obstruction and Jones kicked his second penalty to give the Llanelli outfit a one-point buffer to take into the second half.
The Scarlets, though, fell away early in the second half. Two tries in a seven-minute period after the restart left them chasing the game and gave Cardiff the confidence to see out the result in style.
The writing was on the wall for the Scarlets when prop Filise crashed over after just two minutes.
And the Blues confirmed a bonus point when Roberts breezed in for his second try after Martyn Williams won ball on the ground and it was put through the hands with Kiwi ace Xavier Rush twice involved.
The final nail was put in the Scarlets' coffin when scrum half Rees went over for his side's fifth try on 58 minutes to stretch the lead to 20 points.
With the Scarlets struggling to respond, the final say fell to the Blues who registered a late penalty through full-back Blair.
The win has moved Cardiff up to sixth place, three points ahead of the Newport Gwent Dragons who host Edinburgh on Sunday. The Scarlets remain second-from-bottom in the standings and ten points behind the Dragons with two games remaining.
|Scarlets Score Card|
|Cardiff Blues Score Card|