if its food grade, probably so.

I just found out a couple of minutes ago that McCormick makes a black food dye so there is probably no reason to use the pond dye.

I'd think any of the dyes for icing would work well. I think they use Squid Ink on Iron Chef for their black ice cream. Realize that dying something that's basically "white" with black dye tends to give grey--so you need a LOT of dye to get true black. When I want black icing, I start with dark chocolate--so you might want to do something similar on the ice cream?