CherryTree

I finally figured it out!
 
//image collage outline//
.sqs-block-image .design-layout-collage .image-card {
border:1px solid #000
} 
.sqs-block-image .design-layout-collage .image-card {
background:#f2f1ec!important;
} 
 
That gave me a card background that matched the webpage background, so that I would have some solid overlap. But if you want the entire caption background to be transparent, just replace the background hex code with transparent.