Shouldn't need a blow off for the secondary. A blow off tube is a large diameter tube that you run from your primary down into a pan of sanitized water. It is used in the event that an active beer starts to "blow through" the airlock, or in many cases, blows the airlock out and gushes over. By using a larger tube, you allow the blow off to run out without having to repaint your ceiling (it happens!). By the time you get to secondary, all of your primary fermentation will be complete, and you can use a normal airlock.