on labs.phaser.io has example with shader blur.
https://labs.phaser.io/edit.html?src=src/camera\camera%20blur%20shader.js
When i apply shis hader to hotdog his bg transform from alpha to black
var volcano = this.add.image(400, 300, 'volcano').setAlpha(0.5);
var hotdog = this.add.image(400, 300, 'hotdog').setScrollFactor(0);
hotdog.setPipeline('Custom')
this.cameras.main.setRenderToTexture(customPipeline);
var extracam = this.cameras.add();
this.cameras.main.ignore(hotdog);
extracam.ignore(volcano);