Control the Four Variables
A good cook treats blanching as a matter of control: how much water, what temperature, how much salt, and most importantly — when to stop. Once you understand these four variables, you'll nail both broccoli and potatoes on the first try.
The blanching water should be as salty as seawater: roughly 1 level tbsp of salt per 1 L of water. This lets the flavor reach all the way into the vegetable; too little salt gives a salty outside and bland inside. Keep plenty of water — the temperature shouldn't drop too much once the vegetable goes in.
Drop the vegetable into fully boiling water and keep the lid off. A closed lid traps the acids that break down chlorophyll in the water, turning green vegetables yellow. Add a small amount at a time; too much vegetable at once cools the water suddenly and disrupts even cooking. Leafy vegetables are ready in 2–3 minutes, root vegetables in 8–12 minutes.
The secret of blanching isn't speed but timing: knowing when to stop matters more than how long you boil the water.
Practical Tip
You don't need a kitchen scale to measure the salt: the rule of 1 level tbsp per 1 L of water is enough to memorize.
Once it reaches the texture you want, drain the vegetable and plunge it into ice water. This "shocking" stops the cooking instantly; otherwise the hot vegetable keeps cooking on the plate and turns to mush. Shocking also brightens the green color.
Don't pour out the vegetable blanching water — the minerals and flavor dissolved in it add depth to soups, rice water, or sauces. Legume and mushroom blanching water in particular can stand in for meat stock in vegetarian dishes.