Add the following classes to the Advanced CSS Classes field:
[&_blockquote]:border-l-4
[&_blockquote]:border-surface-600
[&_blockquote]:pl-4
You can customise the border with any Tailwind colour, the example above is using surface-600, you could switch it to the brand colour and change the shade using the number on the end:
[&_blockquote]:border-brand-500
Hope that helps!