Black Tar is basically "half-baked" heroin.
"Real" heroin is made using a chemical called Acetic Anhydride, which easily slaps the two Acetyl groups onto the Morphine Molecule that turn it into Diacetylmorphine (= Heroin).
However, anti-drug agencies are well aware of the importance of AA in producing H, so the Mexicans apparently like to go the easier route and simply use Acetic Acid (hence the smell) and a catalyst to acetylate the morphine. This process, however, is not as efficient as the one using Acetic Anhydride, resulting in plenty of molecules ending up with only one Acetyl group. The funny thing is that the resulting 6-MAM (Monoacetylmorphine) is technically "better" than Diamorphine, being able to enter the brain almost as quickly, but without having to wait for one of the Acetyl groups to be cleaved off before hitting your receptors.