If I may be permitted to revive an old topic here. I have researched this subject for a long time for historical accuracy as I am a craft demonstrator. I use pine pitch all the time in waterproofing my "bottels", I mix it with beeswax. If the stuff you are referring to from Jas Townsend is too brittle then it is probably dried out with much moisture gone, though I think you may be judging this based on its appearance from the bag. When melted down and resolidified it is quite pliable.
As far as "black" pitch is concerned being used for jackware, historically there were several methods. First, common yellow beeswax was used in early vessels going back to the Roman times up through the Middle Ages. There are other references to birch sap when boiled turning black though I do not know of its historical uses. At one point in English history supposedly bitumen pitch which is rendered from coal tar was used, but this is highly carcinogenic.
The real black pitch you are seeking is just regular pine pitch but rendered down over a roaring fire so that it became contaminated with soot to turn it black, I do not know if the process gave it any superiority. You can replicate it by purchasing Jas Townsend's "powdered ink" which they admit is just lampblack (pure carbon soot) and adding it to the melted pitch. Chemically it would be the same thing. By the way, the surviving bottels and costrels from the wreck of the Mary Rose 1535 were laboratory analyzed and determined to have been lined in this pitch.