As with green vegetables, it is crucial that the water maintain a constant boil when you blanch the herbs. The more water you use, the more likely it is that the water will maintain its heat.
Bring a large pot of salted water to a boil. Use about ¼ cup of kosher salt for every quart of water. Place the herbs in a strainer and dip them into the water for the allotted time, keeping the water at a strong boil. Remove the strainer and immediately plunge the blanched herbs into an ice-water bath to chill.