This SVG example illustrates how to apply a merge filter.

MergeFilter.svg

<svg width="400" height="200" xmlns="http://www.w3.org/2000/svg">
  <filter id="idMergeFilter">
    <feFlood flood-color="black" flood-opacity="1" result="imgFlood" />
    <feMerge>
      <feMergeNode in="imgFlood" />
      <feMergeNode in="SourceGraphic" />
    </feMerge>
  </filter>
  <!-- Original Version -->
  <circle cx="100" cy="100" r="85" fill="blue" stroke="aqua" stroke-width="10"></circle>
  <!-- Filtered Version -->
  <g style="filter: url(#idMergeFilter)" transform="translate(200, 0)">
    <circle cx="100" cy="100" r="85" fill="blue" stroke="aqua" stroke-width="10"></circle>
  </g>
</svg>

Output