Mkitzis, (interesting group of sylables you have for a handle, pardner)
My experience tells me you should be ok with these.
You're on the right track, a solution would be prefereble to fingernails. Speaking of fingernails, Acetone (the old kind of fingerenail polish remeover) might work for starters. There's other solutions that might cut it also, as it's probably an acrylic color coat of some kind. But really, the best thing would be to use a leather specific solution with steel wool.
When I read your post first, I thought I was reading snake boots, and when I saw it is either textured cowhide, maybe water buffalo, I was convinced you'll pull these out just fine.
The product you want is Deglazer. There are several different makers of the stuff, of which Feibings is but one. Degalzer should be available from a "shoe finder" maybe a local shoe repair shop can help you get some, or ever pour you a bottle. *Dye Remover by Magix is not the same thing.* I'm told Deglazer has an oil of some kind in it to keep it from drying the leather. So here's what I do to get a good dye job on footwear leathers.
With a 000 steel wool pad, scrub the color off with the deglazer. You'll work up a slurry, so wipe it off with a rag as you work your way around the boot. There's no need to spread it around as you go. After you've worked it off in the first pass, repeat for a thorough deglazing. Next is to apply your spirit dye. Since it looks to be blue leather that was used, you'll get a good final black using Feibings Oil Dye. But if you want to use regular Feibings Leather Dye, I don't think it'll matter, just be certain to wipe all of the "dye stuffs" off immediatly after application.
I don't think it matters if the leather is dry or not from the deglazer. I've done it both ways.
Put the boot up to dry with trees in them, and leave it alone for a day or two. Your job will look way better if you keep the trees in them, cedar or plastic won't matter.
After they are completely dry, now one could use "a color coating". Easiest and smoothest is a leather color spray, again found through shoe finders. My advise is to use as little as possible to get an even coat. Even if you choose a flexible brush-on acrylic, less is more with leather.
Again, let it dry completely, and follow with a cream polish (let it dry before buffing), followed by a wax polish. You should be ready to skate your legs off with beautiful black custom boots.
Good luck,
Paul