Some magic just came to Rumple when he became the Dark One, like the ability to turn others into snails. But the BF did say that his powers weren’t strong enough to cast the Curse and he swore to spend the rest of his life learning. So, either book or he got help. With Cora, he asked because he assumed she had been learning outside of Rumple–again, either by book or with a person. We know he takes power, like the Seer’s ability (even though that was her suggestion) so it’s not out of the realm of possibility that he stole power from others. If there is a story to be told about this, it might come up towards the end of the first half of S3 when we learn how PP and Rumple know each other. It might be that Rumple tried to steal a certain power from PP and got his butt kicked. How scary would that be? PP vs. Rumple and Rumple didn’t win!?