Darken Scene excluding the AR Object

Hey everyone!
I’m wondering how to darken (e.g. 60% grey) my camera view while the ImageTarget is detected but excluding the ARObject (which is text).
I want to provide a better legibility. All I tried so far leads to a darkened screen also darkening my white colored text.

What worked so far is bluring the scene via
GameObject.Find("ARCamera").GetComponent<DepthOfField>().enabled = true;
in the DefaultTrackableEventHandler

Any suggestions?
Thanks!

 

anyone?