To begin with, it is just that: the majority of your rituals, spells, and meditations are done at night. Most of the Nocturnal Witches dress in black, the "Gothic" look, but that really has nothing to do with what they believe, merely how they feel comfortable. The Nocturnal Tradition can be looked at as a sort of darker outlook on Wicca- darker, but not evil.
We aren't your ecstatically happy bunch (at least not most of us), but we aren't eternally depressed, either. We find joy in the night, the time where we connect the most with ourselves and the world around us.
Why do we connect with the night as we do? If we're not evil, then why are we so damn DARK?
There are four soul-types according to the Nocturnal way of thinking. Good-light (Happy, bouncy folks who are what they seem and like to help), Evil-light (The "beautiful people" who are ugly inside, the majority of TV evangelists), Evil-dark (Your very typical villain, mass-murderer, or people that don't bother to mask their evil), and the last, most misunderstood: Good-dark. This is where I come in, and the Children in Black in general.
Our souls gravitate toward death, autumn, rain, and night of course.
When I say death, I am not talking about wishing to inflict it or in any way taking pleasure in the loss of another's life. I'm talking about paying reverence to those lost and the Gods and Goddesses of the Otherworld. Complete acceptance of the inevitable, and even befriending it.
As a result of these unconventional views, Nightkin tend to be very private about their tradition and as a result can be very hard to pick out of society. When they are open about what they believe, they get stereotyped and outcast for the most part- think your high school Goth in the corner, reading quietly by themselves.
We are sometimes outcast even in the Wiccan/Pagan community as they try to rid themselves of the "black witch" stereotype. It grows harder to explain your gods and goddesses... Kali, Hecate, Anubis, Tisiphone... without being looked upon as evil. Most of the gods and goddeses associated with the night and death fall into the Yin, if you will, whereas most Pagans tend toward Yang. Many of them have had bloody cults dedicated to them in the past, so when you say that Kali chose you to be her child, you get a few disapproving looks.
For many, the Nocturnal Tradition is not by any means the way to go. Most are simply drawn toward the light, and there is nothing wrong with that. They simply do not belong in the darkness. But there are some more dangerous follies than simply being in the wrong place.
With Nocturnal Witchcraft, there comes more power than normal (at least this is what I've found). But, like the night itself, it is a fickle thing. You walk a fine line when you use that much power, and with Dark gods and goddesses, it can be tempting to call revenge upon whoever you feel like.
You must avoid doing that at all costs. Even though we are dark, we still follow the Rede to harm none. We converse nightly from spirits ranging from lost souls, gods of war and blood, to Death themself. This brings on a greater responsibility to know what is right and wrong.
There is nothing wrong with summoning someone's own karma upon them (although this is walking a line), but there is definitely something wrong if you're sending out zaps at anyone who looks at you wrong.
We work with what is called the Dark Ether, the energy and life force of the night. It is a palpable presence that you can experience if you merely go outside and "feel". This takes the place of the elements in traditional Wicca, for the most part. We do revere the elements, yet rarely call on them for aid. The Night looks after its children.
The dark ether is simply the darkness, taken on a new form. It is there for you to channel, to take counsel from, to draw peace and serenity from. It is what you feel comfortable in.
We also use something called emotional energy, drawing energy quickly from sensations, images, and memories that gave us either positive or negative feelings. Emotions are energy too. This form of energy raising proves much quicker than traditional techniques I've found.
There is much more to Nocturnal Witchcraft, but I'll leave that to you to discover.