Now that we're dealing with blocks if you want to put a background color on an image you have to go to your advanced, additional CSS classes section and reference a class that you have to create either in your theme files or in your special "additional CSS" area under the customizer.
I needed the background of an image to be black (and the caption text to be white) so I did:
.imgblack {
color: white;
background-color: black;
}

And then referenced that class name in the settings for that particular block.

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