Blocks that allow an image as a background provide the option of adding a semi-transparent layer of color to improve the readability of text over the image.
1. Click on the block, then select the Settings section of the toolbar
2. To select a different color to layer over the background image, choose from your color palette (you can add more colors to your palette on the Theme page)
3. To completely remove the layer of color, toggle "Add color to image" to OFF
4. Click Back to return to the main Settings toolbar