so you sayin that dark bodies eg 266, blackbird, barnard all contain dangers with manganese exposure
Since the suppliers generally will not share what is actually in their bodies, it is impossible to say for sure. But some appear (if you know "what causes what" in ceramics) to contain such materials. If so "yes".
Note (as I mentioned elsewhere in anotehr recent thread) those ASTM "non-toxic labels on most clay boxes (and the info in many MSDSs) apply only to the clay (or glaze) being certified in the WET state. Hard to inhale 25 pounds of plastic clay .
Evaluating all of the H+S stuff depends on your overall potential exposures, and that "look see" has to include the exposures you get from all of the other sources in your life. I spend a bunch of time on this stuff in my ceramic chemistry courses. If you are a hobbiest, and use a total of 100 pounds of wet clay a year...... your situation is very different from a pro that is using 10,000 pounds of clay a year. A peson doing "colored clay" (neriage/nerikomi) kinds of work is using clays that contain lots of colorants.... but they are usually working in a very slow process.... and thereby even if full time........ are not processing the huge amounts of clay per day/week/month that a thrower might. If you are a ceramist that is not exposed to silica dust from any other sources, that is different from if you are ALSO an off-road motorcyclist that is breathing dust all the time. And so on.
To shift toward the gray / black tones in bodies, the usual selection is a manganese bearing compound. High iron will only get you so dark ..... and it tends toward the reddish/brownish tranges of color. Look at earthenwares... for example... high iron bodies.
Cobalt compounds are not as innocous as most potters seem to think they are. I don't know why,...... maybe because they make a "pretty blue" . Check out the H+S info and stuff like MSDSs and the PELs/TLVs for cobalt compounds.
Awareness is the improtant part here, not "panic" . If you know about what you are handling, you can take appropriate precautions and make appropriate informed decisions.