How do I get my plant to stop blackening

That’s almost certainly coco. It’s damn near impossible to overwater in normal circumstances. It’s hungry and I’d guess you haven’t fed it anything yet. That’s my 2 cents.. assuming it’s coco. Not fed at all or coulda been overfed if you have been feeding

You have absolutely no information about this grow in your diary! This looks to be a case of over-fertilization but honestly, you'll need to fill in more information about this grow - otherwise, all you're going to get here in terms of advice will be just plain guesses... and those guesses might be wrong and make even more problems for you. You can edit your diary and fill in more relevant information... so please do that so we can actually know what's been going on and can give you GOOD advice...

High, You give her too much water! A young girl like this maby drinks 50ml a day. This is coco when I see it right? Then you can give 100 ml a day for another week. When her roots become bigger and she becomes more thirsty you can give water till runoff. Happy Growing! 😋