I’m seeing all your post titles in black, both on the homepage and on individual single posts. Here’s a screenshot of what your homepage looks like:

If you’ve clicked on any your newer posts recently to open the full post, the titles do turn brown on the homepage, since brown is your “visited link” colour. That could explain why older posts’ titles still appear black, but newer ones you visited recently are brown.
Here’s an example of a post I just clicked on. After opening the individual post, I returned to the homepage and the title is now brown:

If you’d like visited post titles to remain black instead of turning brown after being visited, adding some custom CSS under Appearance > Customize > CSS could take care of that.
.entry-title a:visited {
color: #000;
}
Just let me know if you need further help with this.