Let's say you have a blog all about designer tshirts and you have this great article called "101 Top Designer Tshirts of 2013". Well, if within the article you want to link out to another site... does it help or hurt at all if the link does not relate to "designer tshirts" or "tshirts" at all?
For instance, I'll read blogs that have a welcoming paragraph which sort of rambles on and on with and might say something like... "this morning I got a cup of coffee, before walking up stairs to decide what tshirt I wanted to wear that day". So, the keyword tshirt is there in all of that but let's say in that paragraph, he links "coffee" to the website of his favorite local coffee shop. How does Google see a link coming from a tshirt blog for the coffee shop? Does it do anything for the coffee shop? Does it hurt him for having a link about coffee on a tshirt blog?
I have a lot of internal links on my blog but my fear is, if I put a link that relates to the content like "tshirts" going to another tshirt site or blog (ie: a competitor), I will only drive traffic to my competition and help my competition's rank.