Roots look good in the photo, but black on the leaves may be from too much water. What kind of soil was it potted in?

the nursery pot has a standard soil mixed with sand I think. How should I get the black off?

Hopefully it is just dscoloration from wet soil. I would try gently wiping off with a tissue or cotton ball, some could rub off. Whatever is left, it will eventually dissaper. If rot, tissue will be soft - I would probably cut that leaf off - but I don't think it is. Repotting into fast draining mix should help - so plant doesn't sit in wet soil. Mixing soil with lots of sifted/rinsed perlite (or pumice, or grit) should help. Try 50/50 mix, or 1:1:1 if using more than 1 ingredient (soil+perlite+grit for example). I would stay away from sand, it is usually too fine and interferes with drainage. When adding any ingredients, mix well, so there are no layers. Adding just a little doesnt really help. Here is photo of soil+perlite mix (from one of our members):