The visitors appeared on course to overcome the loss of three players to the sin bin, with Lewis Rawlins, Steffan Evans and Emryr Phillips all receiving yellow cards, but Cuthbert levelled matters before Patchell added the extras to seal a Blues victory.
Cardiff had extra reason for cheer after four tries through Cory Allen, Dan Fish, Kristian Dacey and Cuthbert guaranteed them a bonus point, with Patchell converting three of the scores and kicking a penalty.
The courageous and injury-ravaged Scarlets scored three tries from Ken Owens, DTH van der Merwe and Gareth Davies, while Aled Thomas converted all three and kicked two penalties.
Scarlets were without international forwards John Barclay and Jake Ball, who were both injured in last week’s game against Ospreys, but they received a boost with the return of scrum-half Gareth Davies.
For Blues, Allen made his first league appearance of the season, with Patchell switching to outside-half to accommodate Fish at full-back.
Tries: Allen, Fish, Dacey, Cuthbert. Cons: Patchell 3. Pens: Patchell.
Fish, Cuthbert, Allen, Lee-Lo, James, Patchell, L. Williams, G. Jenkins, Dacey, Filise, Hoeata, Down, Turnbull, Navidi, Vosawai.
J. Evans for Fish (69), Hobbs for G. Jenkins (64), E. Lewis for Dacey (68), D. Lewis for Filise (53), Reed for Hoeata (70), Cook for Vosawai (64).
Tries: Owens, van der Merwe, G. Davies. Cons: Thomas 3. Pens: Thomas 2.
Collins, S. Evans, Owen, Parkes, van der Merwe, Thomas, G. Davies, R. Evans, Owens, Lee, Price, Rawlins, A. Shingler, E. Phillips, Allen.
King for Owen (69), A. Davies for G. Davies (69), R. Jones for Lee (70), Myhill for E. Phillips (70), Condy for Allen (55).
Ben Whitehouse (Wales).
Ospreys fought off a concerted second-half comeback from Newport Gwent Dragons to clinch a 12-7 PRO12 win and make it two Welsh derby victories out of two in the space of six days.
After a narrow win at Scarlets on St Stephen’s Day, the Ospreys eventually saw off the Dragons outfit by two tries to one, despite failing to score in the second half.
Ospreys, who moved up two places to sixth in the PRO12, were indebted to first-half tries for full-back Dan Evans and wing Hanno Dirksen, while Ed Jackson went over for the Dragons midway through the second half.
The result meant Ospreys extended their record of not having lost to another Welsh side at the Liberty Stadium since October 2010, as Newport, for their part, continue to seek an away win in the competition this season.
Ospreys rested Wales fly-half Dan Biggar, with Sam Davies coming in to replace him as one of five changes from the win against Scarlets last Saturday.
The Dragons also made five changes from the 20-21 defeat by Cardiff Blues last Sunday, though it was Ospreys who enjoyed a high percentage of possession and territory to earn their 12-0 interval lead.
Tries: D. Evans, Dirkson. Cons: Davies.
D. Evans, Hassler, John, J. Matavesi, Dirkson, Davies, Habberfield, Smith, Baldwin, Jarvis, Ashley, Jones, Lydiate, Tipuric, Baker.
Biggar for Davies (65), Bevington for Smith (71), Parry for Baldwin (53), Arhip for Jarvis (59), Thornton for Ashley (80), King for Lydiate (41).
Tries: Jackson. Cons: Tovey.
Meyer, Scott, Hughes, Warren, Hewitt, D. Jones, Pretorius, Stankovich, T. Thomas, Fairbrother, Screech, Landman, Crosswell, Griffiths, Jackson.
C. Davies for Pretorius (46), Price for Stankovich (46), Dee for T. Thomas (71), Harris for Fairbrother (46), Hill for Screech (71), Cudd for Griffiths (46).
Andy Brace (Ireland)