Why did my brown henna paste turn black?

Your brown henna paste may have turned black due to several factors, including the addition of other ingredients like indigo, which can darken the color. Additionally, the paste may have oxidized over time, leading to a darker hue. If the paste was left to sit for too long or exposed to heat, this could also accelerate the darkening process. Lastly, the quality and freshness of the henna powder can impact the final color as well.