I got that same training.. not so much of an LOL when you realize that the reason they teach this BS is that they don't know the truth. Then you believe them and never try henna for 25 years, waste God knows how much money, subject yourself and the environment to horrible dangerous chemicals, ruin your hair and never have the hair you want.
I can't believe I believed them. It's infuriating, actually.
I do understand the reason... that there are companies that call their product Henna but it is not pure henna and has horrible metallic salt dyes in them.. it's these dyes that hurt your hair, not the henna!!!!
You can get black with indigo, but it is a two step process. First you henna, then indigo. A long process for sure.
Semi permanent dyes may do what you want with little damage instead.