The leaves on the one I have which was already here when I moved nearly 14 years ago look purple in the middle, I think Ajuga reptans has purple foliage but can change with light and temperature. At this time of year mine show more purple, when in flower they tend to green out.
A 'stock' photo of Black Scallop, you will find a range of colours on a google search.
http://www.vanbloem.com/index....
There's other cultivars such as Catlin's Giant..
http://www.gardensandplants.co...
Safest to stick with Ajuga reptans.