Perhaps you can guess a kind of meaning based on the word pitch, but it is a very specific definition that I didn’t know.
Pitchblende – a brown to black mineral that consists of massive uraninite, has a distinctive luster, contains radium, and is the chief ore-mineral source of uranium
The sentence, from The Orchard Keeper, describing some tree branches in a pit, wherein a body also lies:
“It took a year’s weather to fret them into the aromatic humus which steeped in what rainwater the pit held and so rendered in turn a tannic liquor dark as pitchblende by which the old man fancied had long been stained the wormscored bones that lay here.”
Jonas David 