Edit in JSFiddle

<div class="container-demo">
  <div class="center block-blue">1</div>
  <div class="block-blue">2</div>
</div>

.center {
  position: absolute;
  top: calc(50% - 50px/2);
  left: calc(50% - 50px/2);
}

.container-demo {
  position: relative;
  width: 300px;
  height: 300px;
  border: 1px solid blue;
  padding: 5px;
}

div.block-blue {
  width: 60px;
  height: 60px;
  background-color: blue;
  color: #fff;
  padding: 10px;
}