Hi it’s not covered by Webflows’ UI at the moment, yet. You can add custom code at site level (in the dashboard, on the custom code tab, head section).
Code like this:
<style>
a:visited {
color: #4a4a4a;
}
</style>
or
<style>
a.classname:visited {
color: #4a4a4a;
}
</style>
(4a4a4a is a color, gray, put your own.)
.classnmae is the name you give to a link or link block. Like this:
http://vincent.polenordstudio.fr/snap/l818k.jpg
So here it would be a.sharing-cta:visited