In my opinion most people do not understand or MISTAKE niches for themes.
Google does NOT know the difference between every niche, but it identifies themes. (see their patents on themes).
To us, a site like
http://www.blackteensfucking.com may be a black sister/lesbian niche but to google (and most search engines) they see it as much more.
This would be a relevant link for the following niches in our eyes.
Black Porn, Black Teens, Blacks Fucking, Teens Fucking, Black Teens Fucking.
But using what google calls as 'themes' we can see that this is actually a relevant link for many different 'themes'.
Black Cocks, Black Porn, Black Teens, Blacks Fucking, Teens Fucking, Teen Porn, Teen Lesbians, Teens, Lesbians Fucking, Hardcore Fucking, Hardcore teens, Hardcore Blacks, Black Pussy, Black Tits, Black Booty and basically ANY term that follows the main theme, or sub theme of the niche. So niches (themes) that revolve around that domains main keyterms (black, teen, fucking).
So a site like
http://www.blackteensfucking.com although it is specifically niched to ebony teens, we can use any of the 3 main keywords or 'themes' and twist out links like this.
I do this on MOST of my sites, and will get relevant links from EVERY site I can find to my blowjobs (for example) site.
It's easy and VERY effective. Because you COULD NOT tell me that
http://www.blackteensfucking.com is not a relevant niche for "Black Blowjobs" or "Teen Blowjobs".
You just have to use your head, and be constructive. But basically, on a large site, you should be able to tie in your theme or niche to almost any other site.