I get that red-on-black color scheme is a nod to the red lighting of a dark room, but the contrast is so low I can hardly read the example screenshots.

Very cool. Is it controlling the plugs via direct Bluetooth connection? Or zigbee via a hub, or wifi (direct or via service)?

I always assumed, probably incorrectly, that there would be too much jitter in the actual timing so I had built a few prototypes of e.g. an esp8266 controlled relay, where you would give the MCU a duration rather than separate on/off commands. But I guess that was wildly over-optimizing. In your case, how stable is the delay from the app sending a command until the switch operates?

I'd add voice mode for color printing in total darkness.

Nice. I have been meaning to do this with an e-ink device but other distractions keep getting in the way.

25$ to unlock all the features...

Good idea. I wonder if the iPhone's camera would be able to measure the projected negative and guide exposure time?

I had no idea what an enlarger was.