I used black oxide in a soap once. I got a pitch black soap with gray lather. :? I probably used way too much oxide, though. About 2 teaspoons in a 1.5 lb batch.
I'll probably try it again in the future using less oxide. I like the look of black soap.
Just a FYI - if you want a black with less likelihood of grey lather, you could try activated charcoal. My white soap with black swirl shows no greying of the lather; my stripey soap has maybe the faintest tinge of grey; my black soap has a definite (but faint) greying of the lather.
I have heard of people grinding up the aquarium stuff. I use the tablets or the liquid Charcodote. I'm not sure how easy it would be to grind the aquarium stuff fine enough, and I can just imagine the mess.
I have never heard of Charcodote, so googled it, and found that it is liquid charcoal used for digestive problems. They also have it in powdered form, for mixing in juice, etc. It is about $1/ounce. Interesting!