How do effects in layers work ?

I've been messing with effects and blend modes recently .

By adding the "source atop" blend mode on an object that is above another one , i get this result ( which is correct ) :

By adding the "source atop" on the layer that contains both objects (and setting the blend mode of both objects to normal) , i get this (nothing) :

My question is : Why changing the blend mode of the layer did nothing to the objects ? How do blend modes work specifically on layers ? I've searched online , but i didn't find any specific description of how blend modes work in layers .

Thanks in advance .

I'm trying to acess all objects within a layer , and change their position .

It would be something like :

"For each object in layer : object.X = object.X -1"

Are you doing this with events or with Javascript?

With events you can use "Object -> Is on layer" condition, but you'll need to repeat this with all objects, or add them to families, for example SpritesFamily, TiledBackgroundsFamily etc.