Create a Reverse Clip-Path – CSS or SVG

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You can put the image above the blue part and you apply the clip-path on it then the result will be the same as if you have created a hole inside the blue part to see the image below:

body {
  width: 100%; 
  height: 100vh;
  padding: 0; margin: 0;
  display: flex;
  }

#box {
  margin: auto;
  position: relative;
  width: 33%;
  height: 200px;
  background: blue;
}

#innerbox {
  background: url(https://picsum.photos/400/400/) center/cover;
  position: absolute;
  inset: 0;
  z-index:1;
  clip-path:polygon(10% 10%, 10% 90%, 90% 50%);
}
<div id="box">
  <div id="innerbox"></div>
</div>

Another idea is to consider multiple background and you will have better support than clip-path and also less of code:

body {
  height: 100vh;
  margin: 0;
  display: flex;
}

#box {
  margin: auto;
  position: relative;
  width: 33%;
  height: 200px;
  background: 
    linear-gradient(to bottom right,#0000 49%,blue 50%) bottom/100% 60%,
    linear-gradient(to top right,#0000 49%,blue 50%) top/100% 60%,
    linear-gradient(blue,blue) left/20% 100%,
    url(https://picsum.photos/400/400/) center/cover;
  background-repeat:no-repeat;
}
<div id="box">
</div>

UPDATE

If you want some opacity, here is an idea where you have to duplicate the content using clip-path (a drawback):

body {
  width: 100%; 
  height: 100vh;
  padding: 0; margin: 0;
  display: flex;
  }

#box {
  margin: auto;
  position: relative;
  width: 33%;
  height: 200px;
  background: blue;
}

#innerbox,#innerbox-2 {
  background: url(https://picsum.photos/400/400/) center/cover;
  position: absolute;
  inset: 0;
  z-index:2;
}
#innerbox {
  /* if you initially planned to have x opacity so you need to set 1-x here*/
  opacity:0.4;
}

#innerbox-2 {
  z-index:1;
  clip-path:polygon(10% 10%, 10% 90%, 90% 50%);
  animation:animate 5s linear alternate infinite;
}

@keyframes animate {
  from {
    clip-path:polygon(10% 10%, 10% 90%, 90% 50%);
  }
  to {
     clip-path:polygon(20% 50%, 90% 50%, 80% 10%);
  }
}
<div id="box">
  <div id="innerbox">
    <h1>Title</h1>
    <p>Some content</p>
  </div>
  <div id="innerbox-2">
    <h1>Title</h1>
    <p>Some content</p>
  </div>
</div>

UPDATE 2

You can consider SVG to do your initial requirement. Simply use an SVG instead of a div where you will have a mask.

body {
  width: 100%; 
  height: 100vh;
  padding: 0; margin: 0;
  display: flex;
  }

#box {
  margin: auto;
  position: relative;
  width: 33%;
  height: 200px;
  background: blue;
  background: url(https://picsum.photos/400/400/) center/cover;
}

#innerbox {
  position: absolute;
  inset: 0;
  z-index:1;
}
<div id="box">
  <svg viewBox="0 0 200 200" id="innerbox" preserveAspectRatio="none">
  <defs>
    <mask id="hole">
      <rect width="100%" height="100%" fill="white"/>
      <!-- the hole defined a polygon -->
      <polygon points="20,20 20,180 180,100 " fill="black"/>
    </mask>
  </defs>
  <!-- create a rect, fill it with the color and apply the above mask -->
  <rect fill="blue" width="100%" height="100%" mask="url(#hole)" />
</svg>
</div>

You can also use the same SVG as background:

body {
  width: 100%; 
  height: 100vh;
  padding: 0; margin: 0;
  display: flex;
  }

#box {
  margin: auto;
  position: relative;
  width: 33%;
  height: 200px;
  background: blue;
  background: url(https://picsum.photos/400/400/) center/cover;
}

#innerbox {
  position: absolute;
  inset: 0;
  z-index:1;
  background:url('data:image/svg+xml;utf8,<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 200 200" preserveAspectRatio="none"><defs><mask id="hole"><rect width="100%" height="100%" fill="white"/> <polygon points="20,20 20,180 180,100 " fill="black"/></mask></defs><rect fill="blue" width="100%" height="100%" mask="url(%23hole)" /></svg>');
}
<div id="box">
  <div id="innerbox"></div>
</div>

Update 3 (what I recommend in 2020)

You can use CSS mask to get the effect you want with mask-composite

body {
  width: 100%; 
  height: 100vh;
  padding: 0; margin: 0;
  display: flex;
  }

#box {
  margin: auto;
  position: relative;
  width: 33%;
  height: 200px;
  background: url(https://picsum.photos/400/400/) center/cover;
}

#innerbox {
  position: absolute;
  inset: 0;
  -webkit-mask:url('data:image/svg+xml;utf8,<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 200 200" preserveAspectRatio="none"><polygon points="20,20 20,180 180,100 " fill="black"/></svg>') 0/100% 100%;
          mask:url('data:image/svg+xml;utf8,<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 200 200" preserveAspectRatio="none"><polygon points="20,20 20,180 180,100 " fill="black"/></svg>') 0/100% 100%;
  background: blue;
}
<div id="box">
  <div id="innerbox"></div>
</div>

And the inverted version using the same shape

body {
  width: 100%; 
  height: 100vh;
  padding: 0; margin: 0;
  display: flex;
  }

#box {
  margin: auto;
  position: relative;
  width: 33%;
  height: 200px;
  background: url(https://picsum.photos/400/400/) center/cover;
}

#innerbox {
  position: absolute;
  inset: 0;
  -webkit-mask:
     url('data:image/svg+xml;utf8,<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 200 200" preserveAspectRatio="none"><polygon points="20,20 20,180 180,100 " fill="black"/></svg>') 0/100% 100%,
     linear-gradient(#fff,#fff);
  -webkit-mask-composite:destination-out;
          mask:
     url('data:image/svg+xml;utf8,<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 200 200" preserveAspectRatio="none"><polygon points="20,20 20,180 180,100 " fill="black"/></svg>') 0/100% 100%,
     linear-gradient(#fff,#fff);
  mask-composite:exclude;  
  background:blue;
}
<div id="box">
  <div id="innerbox"></div>
</div>

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