Hunting for pics that aren’t of eggs for sale.
THese are more bluish: https://www.happiest.net/eggs-hatched-stunningly-revealed-completely-black-chicken/
Black eggs with this article too: https://allgujaratnews.in/en/gujarat-government-serve-black-kadaknath-hen-to-children-food-as-medicine-for-many-diseases/
As the main article pointed out, egg pigmentation is related to chicken pigmentation. So, black egg shells do exist…although they’re no different from any other chicken eggs if you crack them open.