So I want to save the result of blendable material to some texture and use that texture in the further post processing materials to do some rendering logic. But the only way I found is that blendable materials simply write to the main render target.
Hey devast3d -
Assuming that you want the character’s camera to be the resultant image that is saved to texture, you could take a scene capture 2D in your character blueprint and have it attached to your player camera. In the scene capture 2d’s settings add your first Post Process Material then have it render out to a texture. and use that texture in your new post process material which is placed in either your character’s camera or your global post process volume.
Thank You
Eric Ketchum
That would be great if SceneCapture2D would work with blendables, which is not true at the moment 
Postprocessing materials are ignored by the SceneCapture2D component, while other screen effects work.
Yes we are working on this issue (UE-4148, for reference) and I will update you as I know more about a solution.
Thank You
Eric Ketchum
UE4.6 should have this fixed (Blendables work in SceneCaptures). Make sure bCaptureEveryFrame is enabled (it is by default) as it is required for that to work (at the moment, might change)
If you are having problems with using post process materials on Scene Capture Component, change your “Capture Source” parameter to Final Color (LDR) in RGB