You should get this checked out immediately.
If you have - black, very stinky poo that is like liquid tar this is from bleeding high up in your gut - either from an ulcer or varices. This needs to be looked at urgently.
Blood from low down is still red, but from higher up gets partly digested and goes black.
If on the other hand you started taking iron tablets this will also make your poo black (but not stinky).