I got my 2nd mango tree. The first one was a twig and died in two years. This is a big and healthy one. When I repot it into 5:1:1 mix, I noticed that all the roots are kind of black. I understand not all plant roots are shiny white like Gardenia. Citrus roots tends to be brownish, but new roots are white. But this mango looks extremely healthy, dark green leaves, many of them. Doesn't look anything that has roots problem. But all its roots are kind of black, I pinched a couple of fine roots, they are not rot inside. What's going on?