Well, if the wood you are using, takes other colour dyes, you should be able to get some black into them...!
Curtis advice on the 3 steps to get enough dye in the juice mix, is very accurate, so much so that, the last time I was trying to dye something in blue, I started with 50 drops, then went 100, then 200, then I got the ****s and dumped of full dye bottle in a 1 x gallon of juice...!
Did it do the job...??? well, it did a lot better than with the drops counting but, did I even got the blue I wanted, no I didn't, and I doubt that dumping 2 or 3 full bottles of dye would make that much difference, I believe that, the "saturation point" when achieved, that's it, you can get any darker.
So far I used all the dye colours that Curtis sell, and I got identical penetration regardless of colour so, it has to be a reason why you aren't getting any penetration with your black dye...!
Good luck,
Cheers
George