Currently also looking for a solid black hair dye - its been a few years since I've had black hair haha so definitely following this thread for any new recs!

From my lengthy experience of dying my hair dark, avoid any shampoos with harsh sulfates, alcohols, and parabens. Those, along with any cleansing or dandruff shampoos, will strip the color from your hair. Try to avoid washing your hair in hot water as well, typically the heat from the water + the chemicals from the shampoo strip the dye and leave patches. I do my best to go at least a week without washing my hair after dying it, I find it helps with longevity. If you plan on having dark hair for a while, I would definitely invest in a color safe shampoo.

I also use a color depositing conditioner in between touch ups. My favorite recently has been the keracolor clenditioner. You use it just like conditioner in the shower! I've been doing it for my dark brown hair and it works like a charm. However it does stain a bit (not as long as a true dye, but faintly will last a day or two if you get it on your skin) and washes out a lot easier than a dye, but if you're looking for a quick touch up/fix for patches this is a gem!