The blocks may not appear to be on a no plot layer, but you will have to explode them to find out for sure.
If the entities in a block are on a no plot layer (or defpoints) when the block is created, they will stay on the layer, even if you change the layer the block itself is on. If the block is created with the objects on layer "0" then it will inherit the properties of whateve layer is gets placed on.
I would suggest expoding one to find out what is was created on.