How long will heroin last in your system by just touching it?

No extremely unlikely for it to happen anyway for this to happen as many drugs don't because the skin cells block them from entering in the spaces in your skin the only minute possible way it could get into your blood is if you have a open wound etc but even this is highly unlikely. Black tar heroin is normally a low quality heroin in retrospect to its cousins 'China White' and 'Skag' a light brown powder. Black tar heroin can be administered through smoking and injection as indicated by the name it is highly unlikely to be able to snort as it has tar like properties.