Is baby blankets are waterproof or not ?

Why should a baby blanket be waterproof?
Are you going to take the baby out in the rain without any other form for protection than a blanket?
If a blanket is waterproof it probably also would mean that the material does not "breath". That would be bad for the baby as it would sweat underneath the blanket and it cannot be absorbed.
It is very important that our cloth and our sheets and blankets and covers can breath.