This sounds like an issue you really need to get checked out by a professional doctor. There can be so many causes, ranging from 'normal' to 'needs treating early', so please seek medical advice.
Dark blood, at least when it is brown, is usually old blood, and this in itself is normal.
However, with the bleeding every other week and the itching, I am starting to think it could be an infection, if this is the case you would need antibiotics soon as it could get worse.
Another option is that it could be an implantation bleed, meaning you are pregnant. However, this is usually quite light and from your description, I feel your bleeding has been heavier.
Please make an appointment to see a doctor or gynaecologist as soon as you can. It may well be nothing but it really is best to be safe than sorry as some problems will require early treatment.
I wish I could give you more indication of what it could be but to be honest, there are just so many things that have those symptoms, I would not like to make an incorrect guess without knowing all the facts.
I hope all is fine, do let me know!