Compare $tag->term_id
with the value from get_queried_object_id()
. You have to cast the former to integer, because it is provided as a string for no good reason.
<ul id="blog-tags">
<?php
$tags = get_tags();
if ( $tags ) {
foreach ( $tags as $tag ) {
echo '<li>';
if ( (int) $tag->term_id === get_queried_object_id() )
echo "<b>$tag->name</b>";
else
printf(
'<a href="https://wordpress.stackexchange.com/questions/109677/%1$s">%2$s</a>',
get_tag_link( $tag->term_id ),
$tag->name
);
echo '</li>';
}
}
?>
</ul>