Black and grey bodies are made by staining a clay with oxides or commercial stains rather than by finding dark clay in the ground, which is why they cost noticeably more per kilogram than the standard bodies beside them on the same price list. What you get for the money is a surface that needs no glaze to be interesting.
How a body is made dark
Iron, manganese or a body stain is blended into an ordinary clay until it fires dark. The depth of colour depends on how much is added and on the firing: many black bodies fire nearer dark brown or charcoal at the lower end of their range and darkest at the top. The supplier's fired sample and the firing range printed with it are the only reliable guide.
What it does to glazes
Everything on top of a dark body reads differently. A transparent glaze shows the body and looks like a dark gloss; a white glaze over black often breaks grey at the edges; a bright colour is muted unless it is opaque. Many potters use dark bodies precisely for that interaction, glazing only the inside of a vessel and leaving the outside as bare fired clay.
Why it costs more
Stains and oxides are expensive by the kilogram compared with clay, and blending them consistently through a body adds a process step. On Bath Potters' stoneware listing the professional black bodies sit clearly above the plain buff and white rows per kilogram, which is the pattern across suppliers rather than a quirk of one.
Contamination runs one way
A dark stained body will mark everything it touches and the marks show on white work afterwards. Keep separate boards, tools and reclaim buckets, wash the wheel head thoroughly between bodies, and if a studio is shared, agree an order of work. Iron and manganese specks in a white pot cannot be removed after firing.
Questions people ask about black clay
How is black clay made?
By blending oxides or commercial stains into an ordinary body. It is not a naturally black clay, which is why it costs more.
Does black clay stay black at every temperature?
No. Many bodies fire browner at the lower end of their range and darkest at the top. Fire a test tile at your own schedule.
Can I use the same tools for black and white clay?
Not without thorough washing. Dark bodies contaminate light ones and the specks show after firing.