Marley opened the scoring on seven minutes, but Scarlets produced a scintillating three-minute spell as fly half Dan Jones slotted a penalty before grabbing a try, that he converted himself, to give the visitors a 10-5 lead after 35 minutes.
Two penalties from the reliable boot of Jarrod Evans, though, gave Blues a slender 11-10 lead at the break.
Second row Seb Davies put Blues in control with another try on 55 minutes before the game was wrapped up by Marley just after the hour as he grabbed his second of the night.
Blues: 15 Curtis Povey, 14 Josh Rodgers, 13 James Whittingham, 12 Lewis Morgan, 11 Harri Thomas, 10 Jarrod Evans, 9 Carl Lewis; 1 Rob Lewis, 2 Aldi Coverdale, 3 Joel Ringer, 4 Jack Davies, 5 Seb Davies, 6 Jordan Viggers, 7 Matthew Marley (c), 8 Hemi Barnes.
Replacements: 16 Dai Harries, 17 Sam Evans, 18 Kieron Assirratti, 19 Calum Haggett, 20 Owen Jones, 21 Mike Hale, 22 Tyler Williams, 23 Josh Lloyd.