Just in case someone is curious how to actually create a transition-like effect when you are actually changing the source attribute of an image, this was the solution I came up with.
Javascript:
        var bool = false;
        setInterval(() => {
            bool = !bool;
            let imgSrc = bool ? 'hero-bg2.jpg' : 'hero-bg.jpg'; // Toggle image
            $('.parallax-slider').addClass('transitioning-src'); // Add class to begin transition
            setTimeout(() => {
                $('.parallax-slider').attr('src', `https://website.com/images/${imgSrc}`).removeClass('transitioning-src');
            }, 400); // Ensure timeout matches transition time, remove transition class
        }, 6000);
CSS:
.parallax-slider {
    transition: opacity 0.4s ease-in;
    -webkit-transition: opacity 0.4s ease-in;
    -moz-transition: opacity 0.4s ease-in;
    -ms-transition: opacity 0.4s ease-in;
    -o-transition: opacity 0.4s ease-in;
    opacity: 1;
}
.transitioning-src {
    transition: opacity 0.4s ease-out;
    -webkit-transition: opacity 0.4s ease-out;
    -moz-transition: opacity 0.4s ease-out;
    -ms-transition: opacity 0.4s ease-out;
    -o-transition: opacity 0.4s ease-out;
    opacity: 0;
}
This will give the illusion of 'fading to black and back' between images - even if you're using something like parallax.js where you have a data-attribute driven component that renders out into a dynamic image. Hope it helps someone.