Draw it in white on a black background then.
<svg id="svg1" xmlns="http://www.w3.org/2000/svg" version="1.1" width="100%" height="100px" viewBox="0 0 100 100" preserveAspectRatio="none">
<rect width="100" height="40" fill="black"/>
<path d="M0 0 C50 90 100 0 100 0 Z" fill="white"/>
</svg>
or alternatively you could just keep drawing round the edge...
<svg id="svg1" xmlns="http://www.w3.org/2000/svg" version="1.1" width="100%" height="100px" viewBox="0 0 100 100" preserveAspectRatio="none">
<path d="M0 0 C50 90 100 0 100 0 v40 h-100 v-40Z" fill="black"/>
</svg>