| Since no one has chimed in yet, I'll give you my best guess. They could be "Peruvian Purple". They can be very dark, and could appear almost black. Check the photo in the link below, to see if it looks like yours. |
Here is a link that might be useful: Peruvian purple potato
| OK, that helps; my wife is from the Philippines. Your "potato" could be a yam. Does it look like this? |
Here is a link that might be useful: Purple yam (ube)