I know morphine doesn't convert to heroin, but the process of making heroin does convert morphine into heroin.
I misread what you typed. And both types of heroin come from purifying opium to morphine, then acetylating the morphine.. I think tar heroin is just a less refined product of morphine and that's what gives it the darker color and texture.
I found this while searching online. It might give some insight into your question.
'Pure heroin is a white powder with a bitter taste and little odor, but street heroin comes in many different forms, depending on how it was made and what's been added to it. Street heroin can be white, tan, brown, gray, or black.
It can be a fine, fluffy powder, course like sand, chunky, or a solid mass that is either gummy or rock hard (Black Tar Heroin). It can smell like vinegar, vitamins, or medicine—or have no smell at all.No matter what color or form, all heroin is either heroin salt or heroin base.
White powder and black tar heroin are usually heroin salt, and brown heroin is usually heroin base.
Despite all of the different appearances heroin can have, what's really important is how you can take it.
White Powder refers to heroin “salt,” mostly snorted or injected (aka China White, Number 4),
Brown Base refers to heroin “base” (aka Persian, Brown Sugar, Pakistani) that can be smoked but needs to be heated in a solution of water and mild acid to inject and
Tar is the black, sticky, gum-like substance (aka Chiva,Mexican Tar or Black Tar Heroin), mostly smoked or injected.
The heroin you buy on the street, no matter what form, almost always has other stuff in it. Some of these substances are chemicals that come from opium or from the way they make heroin.Others are cuts added to make the dope go farther or change its effect.The cut can change the taste, smell or
high, making it harder to know what you have.
And from another website:
Mexican drug syndicates were producing heroin by the mid-1960s. Lacking the experience in chemistry that other syndicates had gained through years of illicit heroin production, Mexican organizations used less-refined morphine and also substituted primary chemicals in synthesis.The opium from which heroin is ultimately produced is a golden brown-to-black, gummy latex containing an average of 10% morphine, although morphine content can vary from 3 to 20 percent of content. Pure morphine and heroin are both fine white and odorless powders. In order to produce heroin, morphine is extracted from raw opium and reacted with non-glacial acetic acid, primarily acetic anhydride for its efficiency. The purity of the final product, and therefore its color and texture, depend on the purity of the source material. Early black tar heroin was notable for its low purity (usually under 30%, at a time when white powdered heroin from the east coast often tested at over 90% ), but purity levels have increased dramatically as the producers have gained experience. Accordingly, the price per kilogram of black tar heroin has increased from one-tenth that of South American powder heroin in the mid-1990s to between one-half and three-quarters in 2003.
The effects of black tar heroin are identical to those of powder heroin. Because of the consistency of black tar heroin, it is usually injected or smoked. It can also be ground into powder or dissolved in water and snorted.
Mexican brown powder heroin is actually #4 heroin without the addition of activated carbon and the final step of ether and ethyl alcohol used by most #4 producers. As it is often produced by the same syndicates producing Tar, they are considered for practical purposes to be a closely related product. In reality however, they are two different classes of heroin.
taken from
http://www.harmreduction.org/downloads/heroin.pdf
and
http://www.bookrags.com/wiki/Black_tar_heroin
