Make sure you really are just blanching your veggies and not in fact over cooking them? - Not trying to be sassy, I have witnessed family members "blanch" broccoli for around 15 minutes...so gross.

Second the tip to not blanch for too long, then you're just boiling - they should still be almost crisp and vibrantly colored. I've never timed myself, but I don't usually leave anything in the water for more than a couple minutes.

If you want them to stay REALLY vibrant, shock in an ice bath directly out of the blanching water.

Make sure you shock the vegetables immediately in ice water to preserve their color & stop the cooking

Have not had this problem. Maybe, as the others have posted, you're over blanching? Remove from water when they're super bright green and shock them in ice water, or, immediately pour out blanching water, add veggies back on low heat to dry them off, then add your butter, seasonings, etc.

Definitely agree with everyone else, sounds like you're over cooking your vegies, you only need to add a pinch of salt (but not if you're cooking corn as it makes it tough) and add all vegies to boiling water except for potatoes which should be brought to the boil from cold salted water, this helps remove some of the starch.

Several variables come to my mind when folks ask: why do vegetables turn brown when blanched? First, as stated above, the duration of time in the hot water bath matters. Too short and the food is too stiff, too long and the food is cooked and brown, just right and the food is al dente and has a bright color. Secondly, the amount of water in the pot might be too little so that the ratio of acids to alkaline is too great. Too much acid and you'll get brown veggies. You want to easily submerge the veggies, then quickly remove them to an ice bath. Solution: try more water, if that doesn't work, then try --as someone stated above-- adding a bit of baking soda to the water. If you blanch quite a bit of food you'll need to add more water and eventually change the water. Another variable to consider is the cooking liquid, the quality of the water: vegetables will turn brown in an acidic environment, perhaps the liquid, the water (tap water?) that you are cooking in is too acidic. In that case, a bit of baking soda will prolly be your solution, start with a pinch, try it out, if that doesn't work, then add another pinch and so on until you have success. Another variable: you might be on to something-- the type of pot (the materials) and or the condition of the pot could be the culprit, I wouldn't rule it out. If the baking soda trick doesn't work then it could be the pot you are using. Solution: try different pots like an enamel-coated pot, or a high-grade stainless steel pot. Another variable is the size of the pot, a big pot (tall sides) will keep a lot of steam close to the water, and could very well trap volatile acids dropping them back into the water. Try using a skillet, has low sides, so that the steam easily escapes. Just remember to have deep enough water so that the veggies do not touch the bottom of the pan, you want to blanch'm, not cook'm. One last variable comes to mind: the culprit could be over-boiled water. This issue goes under the quality of the water because the oxygen driven out from boiling changes the quality (taste and structure) of the water, which will affect the outcome. Solution: keep the water at a simmer, never boil the water. Okay, well, another thought just came to mind: be sure not to have any oil in the pot, enough of it will trap in water soluble acids that normally escape with the steam. I purposely use oil (or some type of fat) when making a slow-cooked soup or bone broth or veggie broth or stew. The oil on the surface of the water seals in the water soluble flavors (and oxygen), prohibiting them from evaporating.

Well, shoot, I just thought of another variable, here is variable #7 to consider if you are getting brown veggies when blanching: could be that you have too much food in the pot at one time. You'll know you added too much because the temperature of the water will drop, it'll stop simmering and you have to wait a while for the water to come back up to a simmer in order for the food to soften, to become al dente. Solution: blanch just a few pieces at one time, allowing the water temperature to maintain a high heat. The food will cook fast, maintaining the bright color. Again, never boil, just a simmer is best.

Correction to my above post: best to use salt instead of baking soda. I just came across this info on salt and thought it sound advice: http://www.stellaculinary.com/category/site-categories/basic-cooking-techniques/blanching.