Its oral LD50 was tested to be more than 5000mg/kg according to Shepherd Color Company's MSDS for Blue 10G513 (as it is known commercially). That's well into "non-toxic" as categorized by the EPA. For perspective, that's more than ten times less toxic than caffeine.
So, probably pretty safe when used appropriately. I wouldn't go inhaling it, since any fine powder's not great for the lungs, but that shouldn't be news to anyone. Just wear your N95 when mixing dry pigments as you always should.
Isn't LD50 only relevant to acute exposure?[0] Just because you can eat 5 grams of something and survive doesn't mean repeated exposure to it in smaller quantities or in other ways won't hurt you (obviously). E.g. acute inhalation of asbestos has no recorded mortality. I think I'm missing something in your post.
It's not the LD50 for Cobalt Blue, but the LD50 for soluble Cobalt salts is between 150 and 500 mg/kg and Cobalt toxicity seems to be a thing. But the LD50(rat) for cobalt blue is > 10000 mg/kg according to the MSDS I'm looking at. That doesn't sound toxic.
Wiki says this about cobalt: "it causes respiratory problems when inhaled. It also causes skin problems when touched; after nickel and chromium, cobalt is a major cause of contact dermatitis"
So for people looking for an example of something that's somewhat toxic, but much less toxic as a paint. Here you go, Cobalt Blue. The color this new can be substituted for. yeah the toxicities are different, but it's the same idea.
So, probably pretty safe when used appropriately. I wouldn't go inhaling it, since any fine powder's not great for the lungs, but that shouldn't be news to anyone. Just wear your N95 when mixing dry pigments as you always should.