Originally Posted by ratty
It's more likely that the black goo is oil from your oil cooler leaking into your coolant.
Unless someone's put something they shouldn't into the gearbox
or added something like slick50, gearbox fluid should be either ATF red or MTF clear dependant on year (ATF early/MTF late), so it's less likely to be oil/fluid from gearbox. Do you know what's in yours, and if it's been treated with anything? Having dismantled one of these rads recently it's construction means it's more likely to leak fluid at one of the external unions than inside the rad.
How long have you had the car, could anyone have added any kind of leak sealent to the coolant?
otherwise the only black substance I can think of would be as you thought - hose rubber, which I guess means draining it and checking them all. If it is the rubber do you know what coolant was used? if so let us all know so we can avoid it!
Having said all this... ATF is corrosive stuff and if it is getting into the coolant it could be causing the hoses to disintegrate.