[Quoth]

The IR sensors and the plastic have a quite broad response. I'd think slightly more 940mm IR will pass black plastic than 840nm. It's likely only the dispersion angle of the integral lens on the LED has much effect.
Either wavelength works with most IR receivers in VCRs, TVs, Set-boxes etc. Unlike the photo-sensors in touch screens and rotary encoders the IR remote control receivers have a filter to block visible light as part of the plastic encapsulation and use 36 KHz to 40 kHz modulation. The IR receiver has the photo-diode or photo-transistor, a 38 KHz amplifier / filter (to reject 50Hz, 60Hz, 100Hz and 120Hz lighting flicker and sunlight), then a level detector and logic level output. The IR touch screens likely have plain photo-diodes / transistors and the controller IC multiplexed ADC followed by processing. That IC will multiplex the transmit LEDs too. It's likely that it scans slowly with narrrow pulses to save power and then switches to a fast accurate mode when the initial touch is detected. You can play with the sketch application to see that this is very probable.
Also I've designed low power electronics, originally in late 1980s. It was standard to intermittently pulse on the receiver till a signal was detected (battery powered receivers of IR, ultrasonic or RF remote signals).

I've used ANY sort of electronic camera without a professional IR blocking filter to check IR LEDs. Even in 1970s the vidicon tubes in otherwise "solid state" security cameras had no filter to block IR. This was so "heat" lamps with black filters could be used as covert flood lights at night. Some used two 230V halogen spot lamps wiired in series.

Professional video cameras and still cameras have IR and UV blocking filters to avoid false colour. I've never seen any evidence of either in phone cameras, my camcorder or fuji still camera as all those will respond to my UV Led for security marker ink and any IR remote handset.
Some IR handsets use IRDA, a high speed protocol that used to be on some PDAs, phones and laptops. The set-boxes that those use won't work with most universal IR remotes.