Ive also been doing this for awhile now too, but I use a free sbsettings toggle from cydia to enable inverted colors. It is much easier to do this if your negs are sleeved and on a light box than hanging free. You can also rest your phone camera on a loupe for a magnified view.

Sometimes when I do want a quick look while drying, i max out the brightness of the screen (another sbsettings toggle), and open up the web browser and a new blank tab. This gives me a bright white screen to put behind the negs that are hung up to view them quickly.