Nginx sending 301 when using proxy_pass
Either use proxy_redirect or change
proxy_pass http://myblog.wixsite.com/blog/; to
proxy_pass https://myblog.wixsite.com/blog/;
Notice the protocol is https instead of http. Your wix blog is served on HTTPS anyway.
It is not nginx that sends the redirect, it is the website https://myblog.wixsite.com/blog/ that sends the redirect, nginx only proxies the response it gets from the site.