This is most likely a display stretch problem. Try calculating the statistics on your data. If the background and actual image data are very far apart in value, you can get what looks like a 'black and white' image. You might try zooming into an area of image data and stretching on those extents to see if the data will view that way.
Also I am not clear on what you mean by 'borders masked as NaN'. Did you create a mask of the border NaN values? I would instead set the border to the Data Ignore value in the header so these values will be disregarded when displaying and processing. You can do this in ENVI 5.5.x by going to File->Save As and select the input data and mask. In the output parameters, you can set the data ignore value. I don't believe NaN is a valid data ignore value so you may want your background to be a unique value not in your data but not NaN.