This worked for me in 7.1: (shortcut version is at the bottom of this post)
1. Click on menu items as follows:
2. Home menu>Design>Colors>Section Themes
3. Find the Section Theme that's giving you trouble (ex. White Minimal)
4. Click on the pencil icon
5. Scroll to the Image Block where the problem is occurring (ex. Image Block Card)
6. Find "Image overlay" and click on the dot to the right (mine was maroon)
7. A color palette opens. Click on the top left corner of the palette or use the slider at the bottom to make the color white.
8. Wait a second for the change to take place. You should see the dot change to white.
I think you can also do this directly from the image block itself by going to Pages, clicking on the page you want to edit, click Edit, then hover over the block that's giving you problems. Click the pencil icon to edit the block (not the image), then click "Colors" in the pop-up menu. The theme you are currently using should be highlighted. Click on the pencil icon on your current theme, and it takes you to the Section Themes design page and you can follow steps 5-8 above.