Chasing a modest 103, Australia were reeling at nine for three inside four overs but Blackwell (42) and skipper Lanning (30) stitched together an unbeaten partnership to lead the side to the finish line in 18.3 overs.
South Africa chose to bat but managed to put on board just 102 for six with openers -- Dane van Niekerk (45) and Trisha Chetty (34) -- putting on a 72-run stand.
Lauren Cheatle (2/13) and Ellyse Perry (2/13) shared four wickets between them while Kristen Beams /1/18) scalped one batswoman.
Australia made a forgettable start with South African pacer Shabnim Ismail dismissing both the openers -- Elyse Villani (4) and Alysas Healy (5) -- in successive overs.
Ellyse Perry could not even open her account, becoming a victim of Marizanne Kapp.
Blackwell then combined with Jess Jonassen (16) to repair the innings, putting up a 44-run stand for the fourth wicket before the latter was sent back by Dane van Niekerk.
Brief Scores:
South Africa Women: 102 for 6 in 20 overs (D van Niekerk 45, T Chetty 34; L Cheatle 2/13)
Australia Women: 105 for 4 in 18.3 overs. (A Blackwell 42 not out, M Lanning 30 not out; S Ismail 2/15).
