In wet and windy conditions, it was the Irish visitors that took a half-time lead courtesy of a clinical try from wing Matt Healy and the kicking prowess of former Blues man Dan Parks.
But it was the accurate and deadly boot of current No.10 Patchell that ultimately proved the difference as a second half flight-back, inspired by the home pack, saw him kick Blues to a crucial first win of the season.
Much had been made of the new playing surface before kick-off, but supporters were instead looking to the skies come game time as heavy rain lashed the pitch and made conditions hard for both teams.
The visiting pack were the first to flex their muscles as Connacht won successive penalties at the set piece on halfway to give Parks his first shot at goal. However, the former Blues ace was on the edge of his range and saw his long range kick fall short of the crossbar.
Both teams looked to run the ball despite the greasy conditions as the heavy rain continued to fall with all-action Number Eight Robin Copeland impressing as he made big yards down the middle from a 22 drop out.
Connacht back row Willie Faloon was penalised at the ruck but this time it was Patchell who saw his penalty fail to find the target as it cannoned back off the upright and to safety.
It was the visitors who did open the scoring when the Connacht forwards punched holes down the middle before quick ball allowed Healy to collect a pop pass and slide over the line.
Welsh prop Scott Andrews looked to respond immediately for the home side as he collected Dafydd Hewitt’s flat inside pass and burrowed into contact. But the ball again popped loose as Connacht cleared.
Patchell saw a second kick of the night fly left of the target as the game started to mirror the previous week in Glasgow with both teams trading high kicks and looking to force mistakes.
Flanker Andries Pretorius, on his return to the team and on his 50th appearance, forced the next penalty, as he snagged Naoupu at the back of a scrum. Patchell duly found his range with a sweetly struck effort from the halfway line to reduce the lead.
Returning Alex Cuthbert looked to burst into life after a power-packed driving lineout while Parks poor clearing kick put his side under pressure and gave Patchell the easy chance to cut the lead to a point.
The penalty was starting to hurt the visitors, as referee Claudio Blessano spoke to Connacht skipper Michael Swift to warn his players, and it was Patchell who again made them pay with a drop goal for the lead.
Patchell then sparked another break from deep and fed Owen Williams down the wing before Cuthbert broke into space at pace. The Lions star looked to step two men before going to ground as the Arms Park found its voice.
A penalty on the stroke of half-time gave Parks a simple chance to give Connacht a slender 10-9 at half-time.
The Irish side looked to start the second half in explosive fashion and a neat kick over the top was collected by Eoin Griffin who raced into space and fed Healy who was hauled down by Filo Paulo.
The impressive Josh Navidi produced a scintillating run as Blues looked to barrel through tackles down the right, but as play was worked infield the Connacht back row turned over ball at the breakdown and allowed Gavin Duffy time to clear.
Phil Davies made a trio of changes as Benoit Bourrust and Macauley Cook joined the pack and Gavin Evans added to the back line. The forwards duly won a penalty at the next scrum and Patchell nudged Blues back ahead.
Blues looked to cut loose, after another big lineout drive, as Cuthbert arrived on the shoulder of Owen Williams at pace. But as play was switched wide the visitors crowded Blues out and the chance was gone.
The home pack was now beginning to dominate the collisions as they rumbled down the middle. But the home side was made only too aware of the danger of Parks and he saw a penalty attempt drift just left.
Blues again drove up close, after a strong lineout win, before front row duo Matthew Rees and Taufa’ao Filise came up agonisingly close to the line.
Patchell did slot his fourth penalty of the night from wide right and then continued to put Blues further ahead with another successful kick seven minutes later.
Connacht’s night was going from bad to worse as Ah You was sinbinned with five minutes left as the referee finally lost patience with the visitors.
And a high tackle from O’Donohue saw Patchell take his own tally to 21 points as Blues wrapped up a first league win of the campaign.
12m Healy t Park c 0-7
20m Patchell p 3-7
26m Patchell p 6-7
33m Patchell dg 9-7
40m Parks p 9-10
53m Patchell p 12-10
67m Patchell p 15-10
74m Patchell p 18-10
78m Patchell p 21-10
Cardiff Blues: 15 Tom Williams (Robinson 68), 14 Alex Cuthbert, 13 Owen Williams, 12 Dafydd Hewitt (Evans 51), 11 Harry Robinson (Davies 60), 10 Rhys Patchell, 9 Lloyd Williams (Jones 79);1 Taufa'ao Filise (Hobbs 79), 2 Matthew Rees (Dacey 79), 3 Scott Andrews (Bourrust 51), 4 Bradley Davies (Reed 62), 5 Filo Paulo, 6 Andries Pretorius, 7 Josh Navidi, 8 Robin Copeland (Cook 51)
Replacements: 16 Kristian Dacey, 17 Sam Hobbs, 18 Benoit Bourrust, 19 Lou Reed, 20 Macauley Cook, 21 Lewis Jones, 22 Gareth Davies, 23 Gavin Evans.
Connacht Rugby: 15 Gavin Duffy, 14 Teirnan O’Halloran, 13 Robbie Henshaw, 12 Eoin Griffin, 11 Matt Healy (Poolman 64), 10 Dan Parks (Ronaldson 69), 9 Kiernan Marmion (O’Donohue 63); 1 Brett Wilkinson (Ah You 55), 2 Jason Harris-Wright, 3 Nathan White (Loughney 74), 4 Michael Swift (c), 5 Andrew Browne (Muldowney 60), 6 John Muldoon, 7 Willie Faloon, 8 George Naoupu.
Replacements: 16 Sean Henry, 17 Rodney Ah You, 18 Ronan Loughney, 19 Aly Muldowney, 20 Jake Heenan, 21 Paul O’Donohue, 22 Craig Ronaldson, 23 Danie Poolman.