<div class="block-1"></div>
<div class="block-2"></div>
.block-1 {
display: inline-block;
width: 100px;
height: 100px;
background-color: #000;
}
.block-1:hover block-2 {
background-color: #000;
}
.block-2 {
display: inline-block;
width: 100px;
height: 100px;
background-color: #e1e1e1;
}