There is no deficit of 1/1 goblins that cost one mana, but many of them have little or no value beyond being small and inexpensive goblins, and if they can be replaced by another, more powerful card it would be foolhardy to not consider it. But how good does a card have to be to make it worth adding black to a goblin deck? The answer may depend on how much money you have spent on your mana base or how many times protection from red has ruined your day, but in generally I think they would all agree that Knucklebone Witch would make them at least consider it.
Knucklebone witch is a 1/1 goblin shaman that costs one black mana. A rare creature from the Lorwyn expansion set of magic the gathering the ability that makes it worth considering black reads "Whenever a Goblin you control is put into a graveyard from the battlefield, you may put a +1/+1 counter on Knucklebone Witch."
There are naturally creatures which get +1/+1 counters easier, but none of them can be put into play as early or as easily as the knucklebone witch, but it isn't just that this creature is able to get larger it is that it can effect the way the opponent is forced to act. This is a creature that your opponent is going to need to block over some of the larger creatures because once it begins to get larger it is going to become dangerous. For example if you attack with this and a 2/2 goblin creature the opponent is far more likely to block this for two reasons, the first is that when the other goblin dies this will become a 2/2 and the second reason is that this is likely to have an ability that is more vital. This also makes for a card that is a good way to force an opponent to use spot removal before you put out your more expensive goblins.
The real value of this card can come from decks that do not rely on the opponent to kill your goblins. The Goblin Grenade is the best of these choices but there are other ways in which you can sacrifice goblins to gain an advantage as well and if in addition to that you put counters on the knucklebone witch it simply makes the deck that much more efficient.