blur.svg

<svg version="1.1"
  baseProfile="full"
  viewBox="-10 -10 148 148"
  xmlns="http://www.w3.org/2000/svg">
  <style>
    #shadow-seen {
      /*display: none; */
    }
  </style>
  <defs>
    <path d="M 0 32 a 32 32 0 0 1 32 -32 H 96 a 32 32 0 0 1 32 32 V 96 a 20 20 0 1 0 -32 32 H 32 a 32 32 0 0 1 -32 -32 V 32" stroke-linejoin="round" id='path'/>
    <filter id='box-shadow'>
      <feGaussianBlur stdDeviation="6"/>
    </filter>
    <mask id='mask'>
      <rect width="148" height="148" fill='white' x="-10" y="-10"/>
      <use href="#path" fill='black' stroke='white' stroke-width="2"/>
    </mask>
  </defs>
  <rect x="-10" y="-10" width="148" height="148"/>
  <path d="M64 10 V118 l54 -54 l -54 -54 l -54 54 l54 54 m -54 -54 h108" stroke='red' fill='none'/>
  <g mask="url(#mask)">
    <use href="#path" fill='white' id='shadow-seen' filter='url(#box-shadow)'/>
    <use href='#path' fill='white'/>
  </g>

</svg>