I just got a 6 x 10 each pack of colored clear LEDs from CLEDs and was wanting to test them for color and brightness.
I loaded the blink sketch and tied GND and Pin 13 to the power rails on my breadboard. I stuck in a red LED and it flashed on and off and was very bright. I then also stuck in a blue LED but it remained off. I pulled the red LED and the blue one started blinking. I put the red one back in and the blue one stayed off.
Did I reach some sort of cut-off point where the blue LED just refused to start because not enough current?