What is the reason why wool talis katanim have black lines on them? Why don't talis katanim of other materials have them also?
Ashkenazim (those who study the kitzur shulchan aruch) agree that the black stripe design is to remind us of techalet. I posed this question to a member of my shul (Chabad) and got a totally different, yet 100% plausible answer. There are some people from the Chabad movement that claim that the black stripes are a tool to be used to ensure that you can make a bracha on tzitzit. Part of the requirement for the bracha is visual (and it seems that you can use the black stripe and your tzitzit strings to ensure your visability!)