Frozen Vegetables

I'm not much help, but I do know that I always have parboiled such things, just for a minute, before cooling and freezing.
I would generally thaw them before putting in the stew, but I am not sure it's entirely necessary.

I think the advice is to Blanche veg to be frozen
Dunk in boiling water , bring back to the boil , have very cold water in a bowl , with ice drain off the hot water , dunk the veg into the iced water , when cool drain and freeze in a single layer then bag
TBH I think I would just buy frozen veg , unless you want to freeze what you have growing
I only freeze my Sugar Snap peas if I have too many looking like they are getting too big

Overnight may be too long to defrost them, it gives time for them to discolour. They would probably be OK used still frozen - though that could mean the stew takes longer. If you feel the stew would be better with them defrosted, spread them out in a single layer and defrost them in the microwave, just before you use them.

i buy loads of reduced pre.packed veg as i make soup every week (even in hot weather) i just throw them in the freezer and defrost in the fridge, the only ones i cut up are leeks and potatos, i never blanche any of them but as i said i always defrost in the fridge. sometimes i put them in the slow cooker still frozen and have never had a problem.

If I buy more veg (broccoli is a good example) than I can use at once, I blanche them, cool and dry thoroughly (salad spinner helps) and freeze. Alternatively, I make them into soup and freeze it in portions.

I never blanche vegetables. Just chop and freeze and I have never had any problem. I chop and freeze carrots and parsnips from the garden every year and some will be in the freezer for a year or more and I have never had a problem.

I actually googled your problem and nothing came up, so I am at an absolute loss as to what the cause might be.

I’ve never known carrots go black, but peeled and chopped parsnips are the same as potatoes, they will turn dark when exposed to the air, so it's important to cook them right away, or hold them in water with a bit of lemon juice added.

I freeze a lot of allotment veg, always blanch it for one minute after the water has come back to the boil. I no longer plunge the veg in cold water. Instead, I strain the water away (into another saucepan to blanch another batch, no need to use fresh water for each batch) and then spread the veg onto dry tea towels. The residual water dries quickly so the veg is much drier when it is bagged up. The veg cools down quite quickly too.
Our veg has never turned black, no ideas on that, sorry.