Reports

Thinking about the negative margin suggestion by @G-Cyrillus made me come up with a possible solution using a wrapper. I wouldn't call it perfect but maybe it's as good as it gets?

.box {
  top: 10px;
  left: 10px;
  position: absolute;
  border: 5px solid red;
  width: 50px;
  height: 50px;
}

.outer {
  top: 10px;
  left: 10px;
  position: absolute;
  width: 100px;
  height: 100px;
  overflow: hidden;
  border: 1px solid;
}

.blur {
  top: -10px;
  left: -10px;
  position: absolute;
  backdrop-filter: blur(10px);
  width: 110px;
  height: 110px;
}

<div class="box">
</div>

<div class="outer">
  <div class="blur">
  </div>
</div>

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