I believe this is because by default, the site body has a transparent background, so when overscrolling in both directions, you’re scrolling past the content in the body, and therefore showing the color of the browser background.
I’ve used this on one or two sites, but worth doing some testing yourself to make sure it doesn’t have any unintended side effects, but this added to the head of your site, via the Custom Code option should work
<style>
body, html {
background-color: #000; /* Or your desired color */
}
</style>
More info about Custom Code here - Wix Editor: Embedding Custom Code on Your Site | Help Center | Wix.com