You guessed it: A blue cockade.
Here's a picture of Lord Gordon haranguing the crowd. Note the blue ribbon in his hat, as well as the hats of the spectators.
Unfortunately, what started as peaceful protests turned into riots, burning and looting in the city of London. This was only ended when the army was called out to suppress the outbreak.
Fifty Years Later
So we go forward in history and find a similar situation in America. An English-speaking people, the United States, is dealing with an unresponsive government, high taxation, and bills run up by an unpopular war (the War of 1812). Naturally, the blue cockade came to mind.
So the badge of the Nullification Crisis became the blue cockade - a symbol of people pushing back against an overbearing government.