I've seen another thread on here where a guy's dope burnt green when he cooked it up on the foil, but it smoked fine.
it wasn't that long ago either, try the search engine and see what comes up.
Yup...i'm the guy that posted the "Powder heroin briefly turns green when melting" thread. Sadly, I didn't get any helpful answers...I bet somebody out there knows more about this, they probably don't use the forum though haha.
Being that your (OP) situation deals with black tar heroin, while I am using powder heroin...this suggests to me that the source of the green color could originate higher up the drug processing chain than I previously would have thought. Since theres not really any physical characteristics that black tar and powder herin share in common, I don't think that we are dealing with the same cutting agent here...if it is a cutting agent at all?!
Seems like this could either be a leftover chemical from the manufacturing process, because I don't think there are any opioids out there that have the characteristic of melting into a green puddle (except maybe that green methadone haha).
But yeah, I still tend to believe that we are dealing with a cutting agent which can be used to both cut black tar heroin and powder heroin...the agent might be used because of it's bitterness, because it smells good when burnt, or for a sleuth of other reasons. Can you describe the physical characteristics of your black tar heroin a little more OP?....i.e humidity, taste, color, smell. etc.