border-block-color
The border-block-color CSS property defines the color of the logical block borders of an element, which maps to a physical border color depending on the element's writing mode, directionality, and text orientation. It corresponds to the border-top-color and border-bottom-color, or border-right-color and border-left-color property depending on the values defined for writing-mode, direction, and text-orientation.
border-block-color css reference
