For darkening checkering I personally use two different things. A product called trans tint in black color which is a wood dye. It mixes with either water or alcohol for diluting. Or I also use a leather dye in black color called Feibings. I get that from my local farm and livestock store. That is an alchohol based product.
Which I use is dependant on what type of finish is on the wood. If it is effected by alchohol, i use a water based die, if not I use the alchohol based dye. Then I will top off with a clear penetrating sealer.
