That should work, depending on the amount of sun they get. 'Blackie' seems to be darkest in a good amount of sun and sometimes the oxalis wants less but you can see how it goes. You could also consider mixing and matching and also getting Oxalis regnellii 'Atropurpurea' and Ipomoea batatas 'Marguerite' to give the opposite type of coloration, with the purple-leafed oxalis and lime green sweet potato vine.

I say "go for it". anyway, beauty is in the eye of the beholder. I, for one, am partially color blind, so the colors I mix may look good to me, but not to anyone else. Doesn't keep me up nights worrying about it!!! Hee hee