For blog post single page, use the below CSS code.

.blog-single .sh-gallery .slick-prev,
.blog-single .sh-gallery .slick-next {
    color: #000;
}

.blog-single .sh-gallery .slick-prev:hover,
.blog-single .sh-gallery .slick-next:hover {
    background-color: #000;
    color: #fff;
}