For a garment like that, you'd make them and then gradient-dye them -- either start with them just a little into the dye and lower them down progressively, or start with them in the dye as far as you want it to go and pull them out little by little. It's absolutely doable, just tedious and possibly messy; if you can at all, try doing it outside with a bucket and some kind of rig to hold the fabric, because holding it up yourself will get old really quickly. You'll almost certainly want to set it with a dye fixative afterwards to minimise the fading and bleeding.
Fair warning: true black is one of the hardest shades to get to take well. Don't be entirely surprised if it's less vivid than commercially-dyed blacks (though how much depends on your dye, fabric, concentration, etc.). (I dyed the fabric for my Miranda
dress, and it came out a little like a fairly wash-faded black; not with RIT, and it's probably a little extreme because I was going for somewhat faded, but the example serves.
@Nefertieh Glad your stuff finally worked out! (I didn't even think about soda ash breakdown because I've never left things sitting, but that makes total sense ... Dyes usually require something to help them bond to the fabrics, though, whether it's mixed into the powder ahead of time like with RIT or Dylon or added separately like soda ash or acids.)