Sorry to bump but just got done dyeing a blazer and some pants bright purple myself
I don't know much about black, but considering how many little old ladies here in Japan run around with purple hair from dye fading I can believe it'd be hard to use!
I don't have a pot big enough that I would feel ok cooking dye in (maybe it's safe after you wash it but any soup I ever make again would taste funny to me psychosomatically), and washers here don't have hot water, so my first few attempts at dyeing clothes really bombed.
This is what has worked for me:
I use a heavy-duty plastic bucket. First I boil a pot of water and mix the dye (more than it asks for on the instructions) and some salt, stir it (I use the handle of my toilet plunger LOL), put in the fabric (already wet), stir it a bit, and then I use a cold-water setting powder I found in my craft store. The company is called "Best Color," but I'm sure there are other brands that make something similar. It claims to make color take better without heat. So I dump that in and stir it all up, then take the bucket to the shower and put in the rest of the water, put another smaller bucket over it to push everything down into the dye, and come back and stir it every now and then. I leave the clothes in there a LOT longer than the package says just to be sure - if it says 30 mins I'll do an hour or more.
Also, I tend to never try and dye anything any color other than a darker/brighter shade of what it already was, unless it's something really light & neutral like white or beige. Scared of what would come out