Thanks for your answers everybody!
I agree that a soft acrylic would probably be the easiest in terms of being able to throw the blanket into the washer and dryer. But I would prefer to use something that is most if not all natural fiber.
And I've never used superwash wool before. I've heard that even though it says machine washable, that you actually have to be careful (delicate wash, air dry, etc). Is that true? I just want to make sure that I"m giving new parents something that doesn't require special care. They should have enought to do already.