What is the color of clostridium botulinum?

Answer 1

When you have a sharply dented can or metal lid, and you see the BLACK something or other that has affected to inside of the now-sharp metal, seems to be growing on it,
that's when you suspect Botulism.

So I think it's Black. But here: they say in the lab they usually stain it, to see it.

Could the black be rust?

Don't get any donations from Whole Foods either. When I worked there, a worker man was about to put that out for donations after we saw the above conditions. I threw in the trash so hard & made sure it (the jars w/ metal lids) smashed irrecoverably.