I've been looking at whatsthatbug.com but can't say I found it. The markings are a lot like a Monarch's but the colour is very dark.
Yes, it's the Viceroy Butterfly (Limenitis archippus). Often confused with the Monarch butterfly (Danaus plexippus) which is not in the esame family. It's very interesting - evidently the colouring of the Viceroy evolved to offer it the same protection as the Monarch, which gets it protection from eating milkweed, which is poisonous! You have some wonderful butterflies in North America.