Up to Jenkins 1.592, in build history it was possible to distinguish if a link to a specific build was visited or not, because the CSS did not overwrite the default colors for visited/non-visited link.
In 1.593 the links are black, no matter if visited or not.
I request to restore the old behaviour.
This probably takes in war/src/main/webapp/css/style.css
in this block (around line 976) to remove the line color: black;:

.pane.build-name a,
.pane.build-name a:visited {
  color: black;
  text-decoration: underline;
}

or alternatively, to set up different colors for a and a:visited