Is it that cat food is bad for dogs or dog food bad for cats?

I have been told by many people that dogs can eat cat food and its ok but if cats eat dog food theyll go blind. Then, today, the lady who gave me my cats told me that cat food is bad for my dog but it is ok (but not recommended) for cats to eat dog food. Can someone please give me a definite answer so I can act accordingly? My dog loves eating my cats food and I saw one of my cats last night eating my dogs food.