While looking into the schematic of the Nucleo STM32F767ZI, I saw a blue led driven by 3.3V, which is the GPIO voltage of the STM32. The datasheet of this led specifies a typical voltage drop of 3.3V, so how does this configuration work with that 680 limiting resistor, as almost all voltage drops on the led?
On page 5 of the LED datasheet, you'll see a graph plotting forward current vs forward voltage. For the small currents that you need to work with here, it's pretty hard to read, but at very low currents (<1mA) it has roughly a voltage drop of 2.8V. This leaves 0.5V available for R31, so the current will be 0.5V/680R = about 0.7mA. The LED will be pretty faint at that kind of current, but it will work (the luminous intensity vs current graph suggests it will be a little under 2% of full intensity).
Personally, I'd swap that 680R for something a little lower. 470R will bring current over 1mA, which will at least put it actually onto the graphs, and 330R would make significant improvements in the visibility of the indicator.
