Hey Michel,
It is actually by design that the showcase option has a black background because the height of both these columns needs to be the same, otherwise the layout would look very disoriented,
http://take.ms/xtOe5
In your scenario, what you could do is to add a custom css,
[gist]
body #eb .eb-blog-grid .eb-blog-grid-showcase-cover__img {
height: 500px;
}
[/gist]
This way, it would stretch the image.