* {
    padding: 0;
    margin: 0;
    box-sizing: border-box;
}

.contenedor{
    border: 10px solid black;
    width: 400px;
    height: 1500px;
    margin:auto;
}

.caja-azul{
    background: blue;
    width: 100px;
    height: 100px;
    position: absolute;
    top: 0px;
    z-index: -1;
}