Blockonomics has a similar API
https://www.blockonomics.co/views/api.html#newaddress . Its free as well
Its wallet watcher also integrates with the xpub so that you see history, get email notifications of payments
Feel free to mail webmaster at blockonomics.co if you need help with integration
This is so close to what I need, but unfortunately it is not quite exactly what I need. I would rather the callback be HTTP/WebHooks instead of through a WebSocket. I like that I can generate addresses from this system though.