laravel-elixir v6 sass-loader imports

I'm on laravel 5.2 (can't upgrade because I can't upgrade php at the moment). However, I'm trying to use blueprintjs which uses ~ to reference paths. (I understand is a webpack syntax for sass imports), e.g.

@import "~bourbon/app/assets/stylesheets/bourbon";

I've upgraded to elixir v6 to support webpack

    ...
    "devDependencies": {
      "laravel-elixir": "6.0.0-15",
      "laravel-elixir-webpack-official": "^1.0.10"
    }
    ...

But the ~ syntax isn't working for sass import paths.

For example in app.scss:

@import "node_modules/@blueprintjs/core/src/blueprint.scss";

gives

Error: node_modules/@blueprintjs/core/src/common/_font-imports.scss
Error: File to import not found or unreadable: ~bourbon/app/assets/stylesheets/bourbon

gulpfile.js

var elixir = require('laravel-elixir');

elixir(function(mix) {
    mix.sass('app.scss');
});

I assume I need to configure a custom loader in my elixir configuration or something? I'm not sure what vue is, and I don't have anything explicit in my package.json about sass-loader or vue. Should I?