As @pansyface says, the vigorous goat willow roots will escape the pot in their drive to find water. They will also strip out all the nutrients and water needed by your blueberry.
Goat willows are a weed. Water your pot thoroughly, tip it up and take out the entire contents. Carefully pull away the goat willow from the blueberry and chuck it out. Re-plant the blueberry with fresh ericaceous compost and water it well.
Vendée - 20kms from Atlantic coast.
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Goat willows are a weed. Water your pot thoroughly, tip it up and take out the entire contents. Carefully pull away the goat willow from the blueberry and chuck it out. Re-plant the blueberry with fresh ericaceous compost and water it well.