Unless bleach is involved, dark hair usually doesn't pick up or show dyes easily. I don't have dark hair but I remember my hairdresser telling me about a client with black hair who had tried to die her hair red with henna. Because her hair was so dark, the colour didn't seem to show so the woman assumed it hadn't taken therefore didn't tell the hairdresser before the hairdresser put chemicals on her hair to dye it professionally. Apparently it ended in a very bad chemical reaction with the henna which was there just not visible and basically ruined her hair. So it's completely not the question you asked but if you try henna and it DOESN'T show, please be careful using other dyes on it esp in lockdown where pro help isn't available