I was out all day yesterday and didn't get back till this morning, my Beardies had lots of fresh veggies and in their tank and before I left I gave them a few crickets.
However when I got back this morning more than half of my dragon's tail is jet black and looks all tried up (looks like a twig or a stick) and where it starts it's bent. At first it was all covered in sand and looked scaly so I gave her a bath and I can see some colour but it's still majority jet black.
I have no idea what this could be and it all happened within 1 day.. Is this tail rot? Or broken or maybe she got bitten. Or something worse?
Any advice on what I should do would be really helpful.
It sounds like you have more than one bearded dragon in one tank. First off, you have to separate them. They're not like other animals; they don't make friends and they don't get along. They're strictly solitary creatures and there will always be a dominant one that will beat on the submissive one, which can include biting toes and tails. My recommendation would be to put the noninjured one in a new cage, as moving the injured one (if it is indeed a bitten tail) can hinder the healing process because moving it to a new environment can stress it out even more.
If you have a camera, could you post pictures for us to see? It would help greatly. If you're sure it's because of a bite you can put a solution of Betadine and water (diluted to look like weak tea) on the tail.