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 I have used black caligraphy ink which you can buy from a art or Asian goods store. It  can dry out however you can just add hot water and it will be as good as new. You wont have the ant or insect  problems like the sweet stuff. The Japanese caligraphy ink is very shiny and of high quality.

Other things that I have tried is photography "wet plates" in the black glass and metal types.

I have also used Johnson baby shampoo, it has a nice thick consistency from the bottle and reflects well. I once dyed it black and it worked well.  Theres probably endless number of shampoos and dish washing liquid that would work equally well.

Then there is also the genuine black reflective glass which you can still buy. I see this glass was  used in many old survery artificial horizons.

Old dirty tractor oil filled with diesel soot  works well.