If there's lots of sun they can turn from red to purple in a few days, but just being purple doesn't necessarily they are ripe to pick as a couple of extra days in full sun will increase the sugar content and the sweetness. But they can ripen without being actually exposed to full sunlight . We have a couple of thornless blackberries growing up a north facing fence, and another on a south facing fence. Strangely enough, the first ones all seem to ripen around the same time so I suspect temperature also has something tle to do with it.