Our local food distribution site gives what they call sweet potatoes. They are purple looking on the outside with white inside. When I boiled them they got kind of black or green streaks in them. Am I cooking them wrong and are they safe to eat this way? I peel and cut into cubes and boil till fork tender.
A couple of years ago we got some sweet potatoes from an Amish market that we visit whenever we travel along the Eastern Shore of Virginia and Maryland. They were simply labeled "Japanese sweet potatoes". They were as you describe with a purple skin and white flesh. We almost always bake our sweet potatoes and were quite delighted with them as they were very sweet. On a few future visits we asked workers at the market but no one knew the actual variety.
To your question about the color, we mostly buy white flesh sweet potatoes as we find them to be tastier. (Each year around this time we make trips to the Eastern shore to buy boxes of the Hayman variety and some Jersey Whites.) When cooked, white fleshed sweet potatoes do look quite off putting, with varying greenish and grayish coloring. Honestly, if I didn't know what they were supposed to look like I would have asked the same question as you did.
Bottom line is that the color is not so pretty, but a worthwhile trade-off for the flavor!