Add black color to the color list - only if it does not exist in the color list

Hello,

You can try the following

var _doc = app.documents[0];
var _isBlackExists = false;
var _swatches = _doc.swatches;
for (var i = 0; i < _swatches.length; i++) {
    var _swatch = _swatches[i];
    if (_swatch.color.cyan == 0 && _swatch.color.yellow == 0 && _swatch.color.magenta == 0 && _swatch.color.black == 100) {
        _isBlackExists = true;
        break;
    } else if (_swatch.name.toLowerCase() == 'black') {
        _isBlackExists = true;
        break;
    }
}
if (!_isBlackExists) {
    var _blackSwatch = _doc.swatches.add();
    var _color = new CMYKColor();
    _color.cyan = 0;
    _color.yellow = 0;
    _color.magenta = 0;
    _color.black = 100;
    _blackSwatch.color = _color;
    _blackSwatch.name = 'Black';
}