styleBlackKey = function (key) {
        var white_key_width = getWhiteKeyWidth(getTotalWhiteKeys()),
            black_key_width = Math.floor(white_key_width / 2);

        key.el.style.backgroundColor = settings.blackKeyColour;
        key.el.style.border = '1px solid ' + settings.borderColour;
        key.el.style.position = 'absolute';
        key.el.style.left = Math.floor(((white_key_width + 1) * (key.noteNumber + 1)) - (black_key_width / 2)) + 'px';
        key.el.style.width = black_key_width + 'px';
        key.el.style.height = (settings.height / 1.5) + 'px';
        key.el.style.borderRadius = '0 0 3px 3px';
    }
Add styling to individual black key.
@param  {element} el Black key DOM element.