I am trying to compare two DEM products ASTER and SRTM v.3, but the ASTER has the black background which i think will interfere with my analysis. I don't have access to ArcGIS where I can do this easily, I am only working with ENVI 5.2
Can you tell me what module are you using ?
You should be able to do that. You need to use the "Edit ENVI Header" and then open the Edit Attributes. When you are there scroll down to "Data Ignore Value" and set to Zero.
You can open "Edit header" --> "Edit Atrributes" --> "Data Ignore Value", and then set the background value.