It is normal that your' black sesame' has a slight purple color after soaking in water and looks very clear. Black sesame itself contains natural pigments. It is normal for natural pigments to dissolve in water after soaking. If it is dyed with black sesame seeds, the water will be black. So yours is really black sesame, don't worry.

Selecting Sesame: Three Ways to Identify "Dyed" Sesame. A look at the color, dyed black and bright, spotless; The undyed colors are different shades, mixed with individual white sesame seeds. The second is the smell. What is not dyed is sesame, and what is dyed is not only not fragrant, but also may smell of ink. Third, dip a napkin in some water. Normal sesame seeds will not fade. If the paper turns black immediately, it must be dyed with sesame seeds.