I used toner. I embedded magnets, and a recessed gasket channel, so I had to thicken some epoxy for fillets, before laying on the glass. Used West microfibers for thickener. Maybe I didn't use enough toner for that batch, but the thickened epoxy has a bluish tint to it. The unthickened epoxy turned out a nice dark black, though. Add the toner in small amounts, stirring well before adding more. Otherwise it'll clump up.

I like the idea of liquid paint tints from the hardware store. Gotta try that next time.