Perhaps the image looks blank but is not due to the way it is stretched.  If you calculate statistics on the file, does it show that the band has a 0 min, max and mean?  If so, then something went wrong.  If not, then try some image stretch enhancements or bring up the cursor location/value tool to see what and where the actual data values are so you can apply an appropriate stretch.

I had the same issue and it was resolved by stretching after the mask was applied. I'm using ENVI 4.8 and I get a bad stretch with every mask I apply, is there a setting somewhere to change the default stretch? If so, that might be good to make available.

You can set a default stretch for the whole ENVI installation, or for a particular image. To do it for the whole installation, you choose File > Preferences. In the resulting dialog, click the Display Defaults tab. You'll see an area in the middle right of that panel that lets you set the Display Default Stretch. To set a default stretch for one image, right click on the image in the Available Bands List, then choose Edit Header. In the resulting Header Info dialog, click Edit Attributes > Default Stretch. - Peg