dark black furrows in sweet potatoes I have a large jewel yam. I peeled, diced, and boiled it with sugars and some spices. When I peeled it, I noticed there were deep darkish furrows in the potato, like spots where sprouts had been but were knocked off. There were a bit dark around the corner. Against my better judgement, I didn't cut them out. After boiling, the dark black spots seemed to permeate the flesh. It doesn't taste different, but it looks unpleasant. Around them, the flesh is very fibrous. 

Is it safe to eat? Next time should I cut these out or avoid buying potatoes that have them?
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.