Assess the position of your bird table or other feeder. If it is next to the pond, move it to the opposite side of the garden, away from the fish pond. This will encourage the blackbirds to go to the bird table instead of the pond.
Stop giving the blackbirds food for at least a week. Get rid of them by removing the incentive to come to the garden in the first place, and hanging out by your fish pond.
Stand next to the pond when you feed the fish. The blackbirds will be enticed by the food you give your fish, so get rid of them by acting as a human deterrent. Wave your hands and create noise to shoo them away if they come close.
Scoop out leftover fish food from the pond using a fishing net to get rid of blackbirds coming to feed off the food. After you feed the fish, wait until they have eaten and then scoop out the remainder of the food.