Flask: How to use app context inside blueprints?

In your app - when registering the blueprint - you need to push the context manually.

Refer to the snippet below and notice how the call-out to the init_db function is wrapped with the application context - the with ensures that the context is destroyed upon completion of your task.

def create_app():
    app = Flask(__name__)

    with app.app_context():
        init_db()

    return app

Source

From the docs about appcontext:

The application context is what powers the current_app context local

Applied to your example:

from flask import Blueprint, current_app

sample = Blueprint('sample', __name__)

@sample.route('/')
def index():
    x = current_app.config['SOMETHING']

For reference here is a small gist I put together, as mentioned in the comments.