The problem I have with it is that, to me, these are super readable:

    x = [ 1, 2, 3 ]
    y = { 'a': 1, 'b': 2 }

Whereas these are not:

    x = [1, 2, 3]
    y = {'a': 1, 'b': 2}

To me, declaring inline lists and dicts without the leading and trailing space makes it more difficult to see what the data structure is, and what it contains. This is especially true when you have lists or dicts declared inline as arguments to functions, e.g.:

    my_function_call([ 1, 2, 3 ])

versus:

    my_function_call([1, 2, 3])

While it's generally not good style to declare data structures inside function calls, at least with the space it's still somewhat apparent what's happening, whereas without the space it just looks like three positional arguments. I love the idea of black, but in practice I'm not going to use something that changes my own code into something that's more difficult for me to read.