Will this code block in node.js
No

app.post('/register'...


Has a callback and is non-blocking within Node

db.users.save(...


Has a callback and is non-blocking within Node (but will block on your database level, which is normal)

db.users.findOne(...


Has a callback and is non-blocking within Node (but will block on your database level, which is normal)

Your code looks pretty normal. MongoDB has a global lock across an entire mongoDB server daemon, so this is the only level that your current code will block. It will not block on your Node.js level for your concurrent users.