Sofia is a black cat. She looks like a black cat. Her coat is lush and dark, with no trace of rustiness. But when you brush her, and look at the hair up close, you can see her fur isn't black at all, but a very dark brown.
Which is why, I suppose, 'black' is known by several different names in the cat fancy. I have a Tonk and a Birman, both breeds are colourpoint cats so the brown colour is generally a bit lighter anyway... so 'black' doesn't really describe the colour very well. So we end up with different names for it.
In the Tonkinese, it is known as 'brown' in the UK, and in the USA it's known as 'natural'. This is true whether the cat has a solid, tonkinese or pointed coat pattern. In the Birman, it is known as 'seal'.
Cats with the orange making gene have a variety of names too, orange, red, ginger, even marmalade! Tonkinese and Birman cat clubs in the UK call an orange cat 'red', and I've seen some US clubs referring to this as 'flame'.
Of course, in the unpretentious moggy, like my Sofia, 'black' does just fine. But black, natural, brown, seal... they are all referring to the same thing.