If the blue black/wire is grounded (shorted to 12 v -) the lamp will not go out. Check the instrument wire loom to be sure blue/black is not chafed to a neighbor wire or to anything else either. Also check the 2-wire plastic plug at the back of the alternator to be certain it is clean.
If all is well with the signal wire call the local alternator shop and ask whether an internal failure will cause this symptom. If so take the unit out and they will test it, verify what has failed, and fix it.
If the cost to fix is not to your taste - or if you want a far more reliable and useful alternator - this would be a good time to consider a unit like a Compass Marine 100 amp alternator.
http://sbo.sailboatowners.com/gear/gear.php?task=showad&id=5542.
Charles
