<div id="block">
  <p class="text">
  Lorem ipsum dolor sit amet, consectetur adipiscing elit. Nulla vel erat eu risus pharetra lacinia. Morbi consectetur urna vitae ornare fringilla.
  </p>
</div>

#block{
  background:rgba(0,0,0,0.5);  /* 0.5 – 50% непрозрачности */
}
.text {
  color:#fff;
}