I think some people here saying, "Keep it," haven't thought out the consequences of the choices facing the owner of this animal. A handicapped animal needs more care, more time, and, often, more expense than a normal bird. If the person does not have the time, facilities, or means to care for such an animal it is better off being euthanized as a young bird.
Those who have handicapped birds can attest to this. Often handicaps can also lead to reduced quality of life for animals. Unlike humans faced with a handicap, there are no programs or subsidized nursing care for crippled animals. It is a serious commitment.
There is no shame in knowing your limitations or in euthanizing a crippled bird. I have a silkie chick out in the brooder that I have been trying to correct the legs of since hatch three days ago. One leg appears to be unable to bend at all. If there is no improvement in two days I will euthanize the chick. I know I don't have the facilities necessary to care for such a special needs bird. When I am unable to be here all day, I don't want to think about the bird sitting in it's own filth, or crawling (as of now, it cannot walk) through feces.
With a blind animal, it will need additional protection from predation, always need to have a 'seeing eye duck', and it may need to be led to food and water. It is the OP's decision, and not as easy as saying, "Oh, but all cute duckies deserve a chance to live." To cull or not to cull also involves thinking of the animal's future,. and the animal's quality of life.