These are the variables / properties in the default theme for blockquotes:
--blockquote-border-thickness: 2px;
--blockquote-border-color: var(--interactive-accent);
--blockquote-font-style: normal;
--blockquote-color: inherit;
--blockquote-background-color: transparent;
So you could grab the first one and change it to have no left border.
body {
--blockquote-border-thickness: unset;
}