This could be something to do with the output driver, my guess (and hope!) would be that this is on the current source side. That way you actually have a chance to fix it! (The 595 shift registers are on the inside of the PCB sandwich…)
Still you’d have to separate the upper two boards (from the core PCB). Try your best not to bend the long pin headers.
The first and easiest thing to try would be to replace the LED in question. Is it a cPixel or a superflux LED?
The next is to check the soldering to that LED, so
- R23, R25, R26
- IC10, pin 12 (this is the blue output)
- IC11, pins 17 and 18
For IC10 especially, are there any shorts to the adjacent pins? It might be that the output tries to drive into a pin that is “off” and could try to sink current without any limiting. This could result in the +5V supply sagging.
The next thing is to check the 595 registers, but that is a major operation as you need to desolder all of the keys first.