From what I’ve read on other answers, it feels like you’re asking specifically how your ISP is managing to add “blocked.aspx” to the end of your domain. If that’s the case, let’s look at a case study:
You have a web server running on http://mysite.mine/, which a public, completely trustworthy DNS resolves to public IP 10.0.0.1. You can browse to http://mysite.mine/index.aspx or /about.aspx or whatever because you’re hosting it on your server. In reality, it ACTUALLY resolves to http://10.0.0.1/about.aspx because that’s what DNS does – It resolves domain names to IP addresses.
Your ISP has decided that your website needs to be blocked for whatever reason, so they redirect DNS requests asking for http://mysite.mine/ to public IP 192.168.0.1, a webserver hosted by your ISP. So any attempt to access http://mysite.mine/ would actually redirect to http://192.168.0.1/. Once they’ve got that in place, it’s a simple matter to configure their web server to redirect any attempt at accessing that weberver with blocked.aspx. Your browser shows http://mysite.mine/blocked.aspx, but in actuality you’re accessing http://192.168.0.1/block.aspx.
As far as your browser is concerned, it’s displaying http://mysite.mine/, because that’s what the DNS server is telling it, so your browser won’t change the domain in the case of a redirect to the same IP address. This is why you see a blocked.aspx at the end of the domain – Because it isn’t your webserver.