I am trying to identify a vine that grew at my grandmothers some 60 years ago. It was an annual that had to be replanted from seed each year. I saved some seed and over the years have planted it several times but the last time I tryed the seed failed to come up. Now I want to know what it was so that I can secure more seed.

The seed are black with a small white spot at the navel. They are about the size and shape of a small butterbean. The vine has a purple stem and the leaves are a deep green with purple veins in them. I can not remember the color of the blooms but I do remember the flowers grew in small clusters in bunches close to the main vine stem. Any help any of you out there can provide will be most appreciated.