[Alan Anderson]

LEDs don't exactly run off voltage; they run off current. Being a diode, they have a characteristic voltage drop across them, so you do need to have at least that much voltage supplied. But the important part is a current-limiting resistor.

jgannon's analysis is fine. A blue LED at 2.5 volts and 10 milliamps sounds about right. Red or green LEDs usually have a much lower voltage drop, and often like much less current, though some are perfectly happy at up to 25 milliamps or more. If you can, you should look at the specific ratings for the LED you're using.