Oxalis Charmed Velvet looks charming. I think it would look be great with a planting of Japanese forest grass, Hakonechloa macra `Aureola`. For even darker foliage than the Japanese blood grass, Imperata Cylindrica, try black mondo, Ophiopogon planiscapus 'Nigrescens'. I had it planted with Artemisia 'Silver Brocade' and it was one of the most commented upon combinations in my garden. I just love grasses...I mean grass...I mean both