Hello Graham
Firstly, if by BPD you mean Bipolar Disease, then I understand that SSRIs such as Venlafaxine can make it worse, especially if you are not taking a mood stabiliser such as lithium at the same time. This is something that you might want to discuss with your GP, since MH have discharged you.
I was on Venlafaxine for about 9 years (I stopped for a while, but I now take Sertraline), and I also suffered from horrible nightmares, as did a friend of mine who was also on Ven. at the same time. I don't know whether the nightmares are caused by the illness, or by the drugs, but I still have them.
As far as I know, SSRIs don't tend to cause people to vomit black stuff - it sounds as if you have some bleeding from your upper gastrointestinal tract - and you need to take that seriously. Are you also taking any over-the-counter anti-inflammatory (NSAIDs), such as Ibuprofen or aspirin, or prescribed ones, such as Diclofenac? These can cause gastric bleeding, which tends to present as either black, tarry sort of stuff, or something that looks like coffee grounds in the vomit.
I agree with the previous two posts - you need to have this attended to asap. It is important. If you can't bear a trip to A & E, then do at least make an urgent appointment with your GP tomorrow morning, but to be honest, you really need to go to A & E, especially if you haven't had this happen before.
Karen is right, at least you will know that anything serious is being considered, and it will help to put your mind at rest if there is nothing much to worry about.
Ten pm is often a fairly quiet time in A & E on a Sunday (I used to work in one!), so it might be sensible to get yourself there, preferably with a friend to look after you.
I am so sorry that you are still feeling awful despite the med.s.
Please let us know how you get on.
Best wishes
M x