Good Question.....you need to select few longtail keywords which includes "large boxes" and "large blue boxes". For example like "get large boxes", "find large boxes", "Pick large boxes now". By using these long tail keywords you should get back links. Show Once google bot crawl gone through the anchor texts, It assumes that this site is related to large boxes. This is the best way to attain some good rankings for the desired keywords.
See in my view if you will promote "large blue boxes" than you will easily get ranked for large boxes or blue boxes. There is no need to make any term primary or secondary
See in my view if you will promote "large blue boxes" than you will easily get ranked for large boxes or blue boxes. There is no need to make any term primary or secondary