The big black rectangle on the Blog Feed page is because of a missing slideshow. To see this, (using Chrome, Safari, or Firefox with the Firebug plugin) right-click on the black area and select "Inspect Element". This will show you the HTML and CSS code for the page. That blank rectangle is labeled div id="slider". You can get rid of the slider by going to Appearance > Theme Options > Category View: Blog. If you want to keep the slideshow, I'm not sure why it's not there. Maybe you have no posts or no images attached to your posts.
It appears Minimatica does not have an option to change font colors, so you will need to do that in the style.css file.
Instead of making code changes to the Minimatica theme, it is a best practice to create a child theme: http://codex.wordpress.org/Child_Themes. That way you can update Minimatica later and not over-write your changes.
Here's a tutorial on Minimatica: http://photohow2.com/2011/10/minimatica-user-guide-how-to-set-up-a-wordpress-photography-portfolio/