The blue wire is connected through a bulb, the other side of that bulb goes live when the ignition is turned on to "run", as the alternator is not yet turning the stud onto which the blue wire goes is earth, this causes the bulb to light, once turning(engine started) this pole goes live, and of course puts the bulb out as the earth for the bulb dissapears so is important.
However if you get stuck simply rev to around 3500 or so and the alternator will kick in and start charging even without the blue wire on, as it is redundant when running.
The live through the bulb is used to excite the field windings in the alternator to initiate charging(once turning) but without the wire there is enough residual magnetism present in the alt to do the same when revved to around 3500/4000, another quick way is to just flick a live onto the blue wires terminal post once running.
Can check easily put headlights on, and watch against a wall the light will get slightly brighter when revs are raised from idle.
tabetha