Thanks Lisa,
I see exactly what you mean here when you say the background images are appearing blurry. This is happening because the cover control setting is trying to enlarge the photos on its own causing them to become pixelated.
The quickest fix for this in all accounts would be to edit the images in a 3rd Party software and make sure the dimensions are correct before uploading them to your WordPress site. Web applications always have a hard time making images larger without distorting them, especially jpeg files.
You could also try adjusting the height and width of the block your image covers but that won’t work for you if you want your background element to appear full screen.
I hope this helps. Please reach back if you need further assistance.