You could hijack the “highlight” format (ctrl/cmd+alt+h on selected text) within the rich-text editor instead of using a HTML block. Then in your global or post code injection field:
<style>
mark {
background-color: black;
color: black;
transition: color 0.2s, background-color 0.2s;
cursor: pointer;
}
mark:hover {
color: inherit;
background-color: inherit;
}
</style>
There’s no JS needed for this type of hover.