Editing the blade files won't do much on a SPA.
Have you tried this custom less from your admin area?
#app{direction:rtl;}
And in addition to that, you can use a local extender as well, not sure how they co-exist:
<?php
use Flarum\Extend;
use Flarum\Frontend\Document;
return [
(new Extend\Frontend('forum'))
->content(function (Document $document) {
$document->direction = 'rtl';
})
];
