Those are not "blue boxes", that is simply a background color for items that do not have a COVER IMAGE or a CATEGORY IMAGE set.
You can't SET a "Cover Image" via creating Items from an RSS feed as the RSS feed does not import any images as XF Attachments (which a Cover Image is an image Attachment).
What you should do in this case is edit the Category that these items are being created in and set a Category Image (just the image name and extension eg test_image.png, then upload that image to styles/default/xenaddons/showcase/category_images and instead of those blue backgrounds displaying, the Category Image will display.
The only other option would be to edit each one of those and set a cover image (which would require uploading at least 1 image attachment to the item).
OR you could use List View layout type which will use the users avatar in place of no cover image or category image.