Hi Joyce-
If you’d really like to completely hide them, you can do it with this line in your Custom CSS:
body.blog .entry-title { display: none; }
However, that might not be the ideal solution. Your blog post’s titles are pretty important for the overall SEO of your site, and give your readers a better idea what they’d like to click on to read more. If you’d like to simply reduce that margin above and below the titles, you can do this instead:
body.blog .entry-title { margin: 15px 0px; }
The default value for those top/bottom margins is 60px, so you can adjust that number 15px to suit your taste.