Hi,

Please try to add more this code to Customize > Custom CSS to do that:

.wp-block-image figure{
    position: relative;
}
.wp-block-image figure figcaption {
    position: absolute;
    bottom: 0;
    margin: 0;
    padding: 5px 10px;
    background: rgba(0, 0, 0, 0.6);
    color: #fff;
}

Best Regards,

PenciDesign

Hi,

Please go to Appearance → Customize → Custom CSS then add the following custom CSS code:

.wp-block-image {
    position: relative;
}
.wp-block-image figcaption {
    position: absolute;
    bottom: 0;
    right: 0;
    margin: 0;
    padding: 10px 25px;
    background-color: rgba(55, 55, 55, 0.5);
    color: #fff;
    z-index: 9;
}

Regards,
PenciDesign.