Black hairstreak?

I've seen a lot of butterflies like this in the last few days, in the Gorges du Tarn area near Millau in southern France. Today there were at least 30 feeding on a white stronecrop flowering at the side of the road. There seem to be several varieties of hairstreak that look much like this; I'm inclined to think they may be black hairstreaks but happy to be corrected.