If you remove the transparent overlay, the caption will not be readable on slides with a very light colour, so I would suggest limiting the height of the caption overlay instead.
To achieve this you can add the following rule:
.wp-block-jetpack-slideshow_slide.swiper-slide figure figcaption.wp-block-jetpack-slideshow_caption.gallery-caption {
max-height: 2.6em;
}
You can add this to the custom styles panel in the WordPress customizer which you can access from the admin menu:
appearance -> customize -> Additional CSS
That did the trick! Only I found I was only able to place the code in CUSTOM CSS (appearance -> customize -> Custom CSS ). Thanks for your help!