Tar could be 100% Heroin, depending on what your definition of Heroin is. The only difference between tar and powder is that the Mexican drug cartels who cook the stuff skip a step in the traditional Burmese/Afghan manufacturing methods. The end result for both, black tar and brown or white powder, is Heroin #4. Going from memory, Heroin #1 is brown Opium scraped off the poppy pods, Heroin #2 is removal of non-Morphine alkaloids from the Opium/water mixture, Heroin #3 is purified Morphine base, Heroin #4 is the result of acetylized Morphine with,usually, acetic anhydride, Diacetylmorphine. The Mexicans go from step 2 to step 4. They collect Opium, add water, acetic anhydride (or another acetylizing agent) and heat, and the end result is still Heroin #4, it just looks like, black tar.
In that sense, the powder Heroin is 'purer' than the tar. However, the tar Heroin is not cut with other inert or active substances for bulk or effect. The tar Heroin you get on the street is the same tar Heroin that was collected at the end of a cook, it hasn't been stepped on. The powder on the street is going to be cut with other substances, lactose mannitol vitamins etc. In that sense, tar is 'purer'.
If you consider 3-MAM, 6-MAM and Morphine as well as acetylized Opiate alkaloids to be Heroin, then tar can be 100% pure. If you are looking for straight Diacetylmorphine only as being Heroin, it will be much lower. This doesn't mean anything though in terms of potency or subjective effect, as a mix of the above chemicals will provide a rush and high with little difference from straight Diacetylmorphine.