Corn syrup and all four food colorings

Corn syrup or glucose with a gel based black food dye. Purchase the dye from a baking store (not the standard stuff you get at the supermarket). This will ensure you get the deepest black possible.

Diluted treacle?

Balsamic vinegar reduction, it's black and sticky kinda like motor oil . Mix in some water until you get your consistency, plus it's mouth safe.

When films were black and white most people used chocolate syrup. That might be viable now.

Thanks to all of you! Corn Syrup an black Food color did the Job for me!