Juicebox-Pro v1.4.4 introduced the captionBackTopColor configuration option. Please see the Version History for a list of changes between versions.
You can now set captionBackColor and captionBackTopColor independently of each other and the two colors will blend in the middle.
The default value for captionBackColor is rgba(0,0,0,0.3) and the default value for captionBackColor is rgba(0,0,0,0) (fully transparent). (Default values for all configuration options can be found on the Config Options page.)
If you want the entire caption area to have a consistent background, set captionBackTopColor to be the same as captionBackColor.
To replicate the default caption area color of versions prior to v1.4.4, set captionBackTopColor="rgba(0,0,0,0.3)" (black with an opacity value of 0.3) in JuiceboxBuilder-Pro's 'Customize -> Color' section.