Naths, Aghoris, Kapalikas, and others of the Tantric Yoga persuasion, are known for a few similarities of appearance: dressing in black, covering themselves in ashes, and carrying or adorning themselves with skulls or skull-motif accessories. This imagery has made such figures rather popular among certain Western occultists, horror fans, and extreme metalheads. There are even some who call themselves tantrikas who embrace this image for themselves, but who appear to have missed the significance of any of it.
First of all, none of these outward things are strictly necessary. They are good to signal others as to our priorities, but only in cultures which have the necessary context baked into them. Otherwise, they are good as sacraments for ourselves and as reminders of our commitment to awakening. But, along this line of thought, each has its significance.
Black clothing signifies, among other interpretations, mastery over our own tendency toward nescience. Ashes are the remnants of ritual fires; just as we feed offerings into the flames, we feed our experiences into the inner fire of devotion and Yoga. Material ashes are all that remain of everything given to the fire, and they remind us of what remains over as every experience passes away. Similarly, skulls remind us of our own mortality, as well as representing for us the fate to which we ourselves consign our false sense of self. Each of these carries far more meaning than this, but from this we can begin to see the direction of movement they collectively indicate.
One thing must be abundantly clear, by this point: the color black, imagery of death, and the like, are not employed in the Yoga traditions to be edgy or to look tough. They are not meant merely to upset the sensibilities of others, nor does such shallow antinomianism have anything to do with Tantra or Yoga. Instead, we seek by them an intelligent and intentional reevaluation of conditioned patterns. It is the level at which we engage with these things which makes the difference. And by the use or misuses of such symbols, we can also begin to evaluate the offerings of those who would claim to be our teachers.