Hindalium is an alloy of Aluminium, Manganese, Magnesium, Chromium, Silicon etc.
It is safe to continue using a Hindalium kadai after it turns black. The darkening is a natural and normal process that occurs due to the build-up of oils and seasoning from cooking. It actually helps enhance the non-stick properties of the kadai. Just make sure to clean it properly after each use to maintain its longevity.
Aluminium vessels are generally the best choice for cooking hindalium, as both materials have similar properties and conduct heat well. However, it is always recommended to use separate vessels for cooking different types of food to avoid cross-contamination and preserve the quality of the ingredients.