A Ford MUST have a battery warning lamp other wise it will not charge.

The basic circuit is a live ignition feed to the bulb. Then from the bulb it goes down that blue wire to the back of the alternator in which it gets it's earth. Once the alternator has been excited, the voltage will increase on the terminal which the blue wire is connected to, and basically block the return path to earth for the power flowing through the bulb.

You either have a fooked alternator, broken blue wire, bad connection or a blown bulb.

Simple test. Disconnect the blue wire off the alternator and short it to the engine block or similar earthing point. Switch on the ignition. If it comes on then you have a good circuit right up to the alternator meaning that the car is ok but the alternator is fooked. If it still doesn't come on then I suggest you try a bulb and then look at the wiring...