The black color isn’t for aesthetics, it’s serves different purposes. Let me mention a few;
• Black color blocks UV light from penetrating into the water inside. If not, the UV light will cause growth of bacteria and algae in the water inside which makes the water unsafe for consumption.
• Black color absorbs heat and in some countries with colder climate, they make use of the black colored tanks to prevent freezing
I’m sure there are a lot more