No, you do not need to blanch asparagus before roasting it. Blanching is usually done to make a vegetable softer. However, asparagus is a tender veggie and doesn’t need to be blanched. You can blanch it if you want to. To blanch the asparagus you want to boil a pot of water. Then add the asparagus and cook for one to two minutes until they are bright green. Drain the water and then plunge the asparagus into a bowl of ice water to stop it from cooking. Once the asparagus is cooled you can roast it as directed in the recipe.