The answer is quite possibly, it depends on how the proxy is configured and the networking works.
The key to answering this question is the line about the IP address of aaa.com being accessible using telnet or nmap. As this is the case, we can assume that traffic to the site is not being blocked completely.
For example assuming the proxy is blocking based on hostnames it sees (e.g. aaa.com) and not based on other factors (e.g. the IP address) if it doesn't see a blocked address it may not block it.
If you give aaa.com a different address using hosts (which generally takes precedence in name resolution over other mechanisms), the request will go to the web server with that alternate name. Now the webserver may not respond as it will get a request for a host it doesn't recognise and may not respond well to that, or may redirect back to aaa.com which would cause the proxy blocking to kick in.
What we don't know from the question is whether the requests using telnet or nmap were bypassing the proxy and therefore that's why they could get to it, in that circumstance it might not work.
To provide an example, say we're trying to block www.google.com. If you put a line in /etc/hosts specifying one of www.google.com's addresses being www.flooble.com, and then make a request to www.flooble.com and intercept the request with a proxy, what you see is a request to www.flooble.com on the IP address mentioned, with no mention of www.google.com. So if the block is purely based on the hostname, it won't kick in.
Now we know from the question that connections to the web port on that IP address are successful, so it's possible that the connection would work.
EDIT - One other factor to consider, which would be important in this case is "where does the name resolution occur". In some cases with an explicitly configured proxy the name resolution occurs on the proxy, in which case the request wouldn't work. In other cases with a transparent proxy (i.e. the client browser isn't aware that it's using a proxy) the name resolution would occur prior to the proxy seeing the traffic, in which case it would seem from the question that the request would work.