Driving blue led directly from GPIO
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 (&lt;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.