You can create a solid color box in an imaging program like Photoshop (or whatever you use). Save it as a .jpg or .png. Then in Edit Layout in Blurb, create an image box in a size that would fit behind your text, just slightly larger than the text box  (and make sure the text box is in front). Then just drag your solid color box into that image container when you’re back in Edit Book mode. If you want a semi-transparent box behind your text, you can create that in your imaging program by adjusting the transparency of the layer, but you have to save it as a .png, as .jpg’s don’t support transparency.