Sandy, and acidic soil will mimic its native environment. This means that it will be in its ideal condition for growth, however, they can do just fine in just about any soil. 6.5 is just fine, and can even be brought to 6 and it would be just fine. I would stay away from alkaline soils. Although they can grow in it, this would not be ideal.
Here is an article for more information: https://www.gardeningknowhow.com/ornamental/flowers/butterfly-pea/planting-butterfly-pea-flowers.htm