I ran out of black pigment and need to do a black singlefin box. Is there any way to mix a black tint with a bit of white pigment to get black pigment. Anyone tried this?

Try to get some black paint (not waterbased) and try it with a small amount of resin.I had the same probelm and worked for me.If you mix black and white you'll get a grey pigment of course unless you can do a miracle!

If your board will have a pinstripe, say red, then use red resin to set the box,    It is a nice cosmetic touch.  I've done it numerous times.

Get some charcoal, crush it into fine powder, mix with resin…

I’ve used Printer Ink with epoxy… works pretty well, but you’ll need to use a lot to get it opaque.

Firstly, as the Thrai master has spoken, you don’t need to set box with a black color.  It can be any or none.  I prefer white.  Nonetheless, by mixing the primary colors of red , yellow, and blue together, it is dark enough to pass for black.  Also mixing any two mixing secondary colors will produce a dark enough color to pass for black.  I really did learn this in kindergarten.  I guess it is true, “everything you need to know you can learn in kindergarten”.