Is it possible to call Black as an API?

You could try using format_str:

from black import format_str, FileMode
res = format_str("some python code", mode=FileMode())
print(res)

Use black.format_file_contents.

e.g.

import black

mode = black.FileMode()
fast = False
out = black.format_file_contents("some python code", fast, mode)

https://github.com/psf/black/blob/19.3b0/black.py#L642