Are there imposter blueberry bushes?
I found a separate growth in my blueberry bush barrels that seems to copy the blueberry leaves although the fruit is strange. A reddish flower with a single dark blue (droplet) fruit.
It's probably a sport, that is a branch that has reverted to the characteristics of a progenitor plant, or it could be a sucker growing from rootstock, if your plant is grafted. In either case, you can reach into the plant with pruners, and cut off the branch bearing the strange fruit where it emerges from the root or a larger branch. While you're here, an article on growing berries in containers: https://www.gardeningknowhow.com/edible/fruits/berries/berries-growing-container.htm