Can I add this Hypothesis JS embed?

Yes, it works nicely. I don't think it taxes Blot as it's merely calling a script hosted elsewhere.

I used the following code to change the colours and fonts of the hypothesis sidebar to match my theme.

 <script>
window.hypothesisConfig = function () {
  return {
    branding: {
      appBackgroundColor: '#FEFFFB',
      ctaBackgroundColor: '#3822BB',
      ctaTextColor: '#eee',
      selectionFontFamily: 'helvetica, arial, sans serif'
    }
  };
};
</script>
<script async src="https://hypothes.is/embed.js"></script>

PS. I decided against having this on my blog because I prefer Memex Go

It definitely won't tax Blot! Glad you got it working