What is the black goo leaking onto my alternator?

When you say "goo" it makes me think of a somewhat thick substance (not like oil). I don't know the engine diagram right off, but it could be power steering fluid (a little thicker than oil). You may have a power steering line loose. Now, you could notice that the vehicle may be harder to turn at speed or at stop, but the leak may be small so you don't notice it. Or it may not be that. It might also be brake fluid (much thicker than oil). You could have a brake line loose (that you may or may not notice) or the brake lines may have air in them and may need to be bled (but you'd notice that (mushy pedal feel, longer stop distance). I'd start w/ those two areas. Your owner's manual should give you a diagram of the engine and you could see what is close to the alternator (power steering lines/resovoir or brake lines, brake resovior, brake master cylinder). Hope this helps.