Jack Be Nimble is a character that only appeared in Blue's Big Treasure Hunt. He is voiced by the late Gregory Hines (who also voices Big Bill in Little Bill, another Nick Jr. series).
Jack jumps past Steve and Blue as their clue from Little Miss Muffet led them to him. They looked for him in the well (Jill's area on the left), and the beanstalk (the Giant's area on the right), but couldn't find him. Jack appears and has a problem. The nursery rhyme starts with "Jack be nimble, Jack be quick". Steve and Blue had to figure out what Jack had to jump over. On a tree stump, there was a lamp, a candlestick, and a pail of water (which is Jill's favorite thing) to choose from. The answer was a candlestick. The whole nursery rhyme is: "Jack be nimble, jack be quick, Jack jump over the candlestick." Jack had a scroll for Steve and Blue to thank him for his help.