dark black furrows in sweet potatoes
Without pictures (or perhaps lab results), it is very difficult to say.  It sounds like your sweet potatoes may have been infested with mold (thus the dark color), in which case the wisest course of action would be to have discarded them.

In general, you should be suspicious of any unexpected appearance in your produce.

Note:  I am inferring the item in question really is a sweet potato, marketed as a yam.  True yams may have channels inside, and are a very different vegetable.