Creating a quality backlink for a "unrelated" subject

If I own a "marketing niche blog" writing articles about marketing and things related to the business etc.. I get a lot of incoming good PR niche links because it's a good blog on marketing etc.

Next thing I write a post about something unrelated (maybe Lego), to make it more natural written from a marketing perspective

And in that article I link to my lego website

Is that a really good backlink or should the background of the Lego link be more weighted towards Lego to be really effective?

I understand it will be a decent link but will there be a noticeable difference between this and a link from a site that is much more heavily affilated with lego?

I think so.. The link from the site that is heavily affiliated with lego will have more quality and power than from a random article.

Thnx for your input Tyrannis. You mean from an isolated article on "lego" right?

But it will still be a superior backlink to an article article on "random"?

I wish I understood a little better how the background of a backlink weighs into its strength.
Do you have any guess how they relate(however vaguely) in quality and power?