so that the user will never leave it.
As a user, it is MY choice to leave a site. Always.
(My website isn’t even properly Live yet so I need to keep as many visitors on it for as long as I can without leaving too early).
Relinquish control today. Users decide how long they stay on a site. You can work very hard to force people, but it will not benefit them. In the old days, people’s CPUs would be working overtime running several instances of their browsers. Then Opera invented tabbed browsing (if not Opera, then some very obscure browser first), everyone else copied it, and now instead of having several copies of the browser open, people can have one browser open with many tabs. This is nicer for most people.
Okay, what you’re talking about when you say “So it just looks as though the blog website has taken over my web-site” is you mean, the link works like normal links do. This is what most (experienced) web surfers expect when they click a link: that they go to a new page in the same browser window, and to go back they click the Back button.
What you seem to want I think is a frame or iFrame within your page. Or the blog to open in a new window so the old window with your page remains. Uniformly a bad idea, but both can be done.
However consider SitePoint. There is a main website, SitePoint.com, with I dunno, articles and stuff on it. If the user clicks “Forums”, they are brought here. The Forums page completely replaces the SitePoint home page. The top looks similar, though, because SitePoint owns both their home page AND the forums, so on the forums pages they put the same menu on top as what you see on the main page (so a user could go back to the main SitePoint page from the menu here on the forums).
Do you own/control fishing.my-forums.net? If you do, you can have that page also have a menu with your home site in it, like SitePoint does.
If you don’t have control, and insist on having both pages show at the same time, you are probably going to have to use iFrames. I wouldn’t recommend this, but it isn’t uncommon (usually, a web site has something like a live weather map on it which actually is from another server, so a completely different address/URL, but sitting on the main page as if it were part of it). You’ll want to make sure that your doctype at the top of your home page’s HTML code matches the doctype at the top of the fishing.my-forums.net’s HTML code. Otherwise IE9 has been showing some nasty behaviour when they don’t match. If you don’t control fishing.my-forums.net, then you’d need to copy their doctype since obviously you can’t change theirs.
This is an interesting post and the comments by readers about how they use tabs (as opposed to new windows) are also enlightening. I think I’m going to post that over in Accessibility.