re: Anyone know how to make epoxy solid black in colour?

Moser’s dye works really well but will leave a translucent effect. The OP wanted a solid black epoxy to fill a crack in a plank.

I’ve done some experimenting with what the OP is trying to do. The trick is to get the color and finish you’re after without adding too much of a tint color. I’ve tried various method where I got the color I was after, but needed to add a lot of tint to get there. The results are usually a tacky finish that could not be sanded to the high polished look I was after.