Link exchange is not only part of Black hat SEO.It is also in white hat and gray hat SEO.
Link exchange is a method of trading links with other web sites that means exchanging the url between two websites.
why to go for link exchange. go the right way. it will take some time but it is controllable. always check for the backlink graph. graph goes down freeze the referring domains and increase likning. while in link exchange it might not be so controllable. As soon as the pr will go up it may come down with the same speed.