The image box element, when you set a custom font colour and add a link, the custom font colour is ignored as there is no css style referencing the element of the class for the widget. So it default to blue or the theme default

The below class controls the font settings applied to the Title of the image element without a link, to render this class when a link is added you need to add an additional style for the

Current:
.elementor-image-box-content .elementor-image-box-title

Required:
.elementor-image-box-content .elementor-image-box-title a