Lots of the early colonial descriptions of the natives referred to some groups with light hair and even occasional blue eyes. Such groups were described especially in Florida though the mid Atlanti region produced such descriptions also. Read the early, pre disappearance, descriptions of the Mandans.
Prince Madoc supposedly arrived in America around 562 AD. It would not surprise me if some Indians took them in. I have read of Welsh settlements in what is now Louisville and Ohio River Valley. There is suppose to be an Irish tribe in America that got marooned around the 8th century living in what is now Tennessee and Carolinas.
Thomas Jefferson sent Lewis and Clark on their expeditions to the western US and I've read that Jefferson stopped them when they began to describe the Mandans. He said, "I don't want to hear any stories about blue-eyed Indians." The writer of the article speculated that Jefferson feared that another European power may try to lay claim to that area in the US based on these stories about European 'looking' people. Some even said they spoke Welsh.