Several places sell dyes to darken the water in ornamental ponds and reduce algae. Sellers describe them as being safe for fish and humans. Some describe them as being “food-grade” dyes. One example Pond Dyes - Aquashade has a black dye. Could one use this dye to make black ice cream?
I don’t know about pond dye but McCormick makes black food coloring. I’m sure if you searched around you could find what you need without having to use pond dye.
I’d say the problem with making black ice cream is that ice cream is white. Any black dye you use is going to be “diluted” to grey.
There are some powdered food colorings available, they might work better than liquids. I wouldn’t risk the pond dye, plus, they probably taste foul.
And if you use enough to make it really black it will probably dye your skin, your mouth, and your shit just as black
I used the McCormick’s black dye in a cake. Everyone could tell who did and who didn’t have any. If you insist, I would recommend using things like chocolate, ground oreos, blackberries, and other dark ingredients to start out with, and use the black to enhance that.
Another option is to use very red items, then use green food coloring to darken it.
My mom used to have a little side business making cakes, and once in a while she’d get asked for one with black decorations (usually for a 40th birthday or something) and wow, trying to get frosting to turn black instead of just gray, not easy, and wow does it ever leave coloring all over your mouth (or anything else that it touches).
I can only imagine trying it with ice cream would be similarly not worth it. Unless you’re a goth with a sweet tooth, maybe.
I made an Elmo cake for my daughter and used some Wilton food dye to make his mouth black. Even using the correct stuff it tasted nasty.