It depends on if the layer is drawing off-screen or not. By default everything just draws directly to the screen. This means all effects and blend modes work as if everything was on the same layer (so you can blend across layers).
However if the layer has 'Force own texture' enabled, OR it uses layer effects or a layer blend mode, OR its opacity is not 100, the layer will draw to its own off-screen surface. This means objects on the layer can't blend with any other objects from other layers. However as long as you don't use any of those things, objects should blend between layers just fine.